We must, however, beware of simply transferring to the Mesozoic world the kinds of Monotremes and Marsupials which we know in nature to-day.
It seems clear, at least, that two main branches, the Monotremes and Marsupials, arose from the primitive mammalian root.
The Ornithosauria have long passed away; the Monotremes are nearing extinction.
The monotremes are pure oviparous animals, like birds and reptiles.
The monotremesare found nowhere but in Australia and the neighboring New Guinea.
The non-placentals include only the marsupials and the monotremes (ornithorhyncus and echidna).
The monotremes actually lay eggs and incubate them.
This portion is alone found in Monotremes and Marsupials.
Huxley holds that the so-called marsupial bones ofMonotremes and Marsupials, which as shewn by Gegenbaur (No.
Hence I had, in 1875 (in my essay on The Gastrula and Ovum-segmentation of Animals), counted themonotremes among the discoblastic vertebrates.
In the monotremes more primitive conditions have been retained better than in the reptiles and birds.
The two lower sub-classes, monotremes and marsupials, retain the simpler structure of their ancestors, the reptiles.
But in the monotremes the formation of the cenogenetic entoderm does not precede the invagination; hence in this case the construction of the germinal layers is less modified than in the other amniota.
In this way the original food-yelk of the monotremes gradually atrophied, and at last disappeared so completely that the partial ovum-segmentation of their descendants, the rest of the mammals, once more became total.
The persistence of the cloaca is not a sign of primitivity, since one finds it in selacians, batrachians, reptiles, monotremes and birds.
The monotremes of the Triassic period were followed by the marsupials of the Jurassic, and these by the placentals of the Cretaceous.
In the monotremes the mammae are looked upon, not as modified sebaceous glands, as in other Mammals, but as altered sweat glands.
Adult monotremes are in like case, although the duck-billed platypus (Ornithorhynchus) has teeth when young on the sides of the jaws.
In the monotremes the mammary pit remains throughout life, and the milk is conducted along the hairs to the young, but in other Mammals nipples are formed in one of two ways.
In the pouched Mammals (Monotremes and Marsupials) inguinal mammae are found, and so they are in most Ungulates as well as in the Cetacea.
With the exception of the atlas of Echidna the cervical vertebrae of Monotremes are without zygapophyses.
In Monotremes the transverse processes ossify from centres distinct from that forming the body, and remain suturally connected with the rest of the vertebra until the adult condition is reached.
A primary division of the Mammalia, including the monotremes and marsupials, in which no placenta is formed.
A division ofmonotremes which comprises the spiny ant-eaters of Australia and New Guinea.
The higher mammals had appeared, and henceforth the lower mammals--the monotremes and the marsupials--are reduced to a subordinate place.
Its independence from the epistropheus is emphasised in Monotremes and some Marsupials by its late fusion with that vertebra.
So long as it was believed that the mammary pouch of the Monotremes was the equivalent of the marsupium of the Marsupials, the persistence of this structure seemed to be a bond of union between the groups.
Prolongation of the pouch and reduction in caliber lead to the formation of the slender lateral caecum found in all the Monotremes (Figs.
Only in the monotremes do the anus and the genito-urinary tract open into a common cloaca surrounded by a sphincter common to the anal and genito-urinary openings (sphincter cloacae).
The placenta, as just stated, is absent in the monotremesand is only slightly developed in marsupials, in which animals the foetus develops to maturity in the marsupial pouch after leaving the uterus.
The two genera of Monotremes produce their young from eggs hatched outside the body; Tachyglossus lays one egg which it carries in an external pouch, while Ornithorhynchus deposits two eggs in its burrow.
In all mammals except the Monotremes they discharge their product through the paired ureters into a bladder, whence the urine passes from the body by a single median urethra.
Defn: A division of monotremeswhich comprises the spiny ant-eaters of Australia and New Guinea.
Defn: A primary division of the Mammalia, including the monotremes and marsupials, in which no placenta is formed.
It was not until 1884 that it was conclusively proved that the Monotremes did actually lay eggs similar in structure to those of birds and reptiles.
Indeed, with the exception of the still more remarkable monotremes [q.
The Monotremes are strictly confined to Australia, Tasmania, and New Guinea.
Scientists are divided in theories as to whether the Monotremes are actually descended directly from the Reptiles or Birds, or whether there was a common ancestor from which Reptiles and Birds and Mammals branched off.
And the Monotremes are certainly one of the surviving forms of the intermediate stages.
It was proved by Haacke and Caldwell in 1884 that the Monotremes lay large eggs like the reptiles, while all the other Mammals are viviparous.
From the comparative anatomy and ontogeny of the existing Marsupials we may draw very interesting conclusions as to their intermediate position between the earlier Monotremes and the later Placentals.
But this primary arrangement is retained permanently only in the Monotremes (and the lower Vertebrates).
It is then clear that the Marsupials--viviparous Mammals without placenta--are a necessary transition from the oviparous Monotremes to the higher Placentals with chorion-villi.
Like these, the Monotremes have a strongly developed caracoideum.
This is the case also with the lowest mammals, the oviparous Monotremes and most of the Marsupials.
In them the milk comes out through a flat portion of the ventral skin that is pierced like a sieve, as we still find in the lowest living mammals, the oviparous Monotremes of Australia.
From these and other less prominent characteristics it follows absolutely that the Monotremes occupy the lowest place among the Mammals, and represent a transitional group between the Tocosauria and the rest of the Mammals.
Further, the brain of the Monotremes is very little advanced.
There is still greater advance when we come to the Mammals, though even here the minds of the Monotremes and of the stupid Marsupials remain at a low stage.
During the Triassic and Jurassic periods the sub-class of the Monotremes was represented by a number of different stem-mammals.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "monotremes" in a variety of sentences. We hope that you will now be able to make sentences using this word.
