As to whether they represent the remnant of a once present system of epidermal scales, which may have preceded the coating of placoid elements in the evolution of the vertebrate, there is no evidence.
An important feature is the complete absence of all trace of the calcified placoid plates which are so characteristic of the Elasmobranchii.
In the line of descent the placoid scale preceded the ganoid, which in turn was followed by the cycloid and lastly by the ctenoid scale.
Placoid scales are ossified papillae, minute, enamelled, and close-set, forming a fine shagreen.
The skin is covered with placoid scales, or shagreen, or with bony bucklers, or else it is naked.
The other element is originally formed by the fusion of the ossified bases of the dermal placoid scales already described in Chapter XIV.
Hairs differ in their mode of formation from scales in a manner analogous to that in which the teeth differ from ordinary placoid scales; i.
The most primitive form of the first type of dermal structures is that of the placoid scales of Elasmobranchii[150].
They pass by quite insensible gradations into normal placoid scales, such as cover the general surface of the body.
In some cases the placoid scales are enlarged to form plates or spines capped or coated with enamel.
The plates or scales of many Ganoids may have been formed by the gradual fusion of elements similar to these placoid scales, and often bear a number of little tooth-like processes.
Ganoid= scales are flat plates of bone coated with an enamel-like substance, and articulating together with a peg and socket arrangement; they are probably identical with enlarged and flattened placoid scales.
The teeth of Selachii are fundamentally identical with placoid scales.
The exoskeleton is in the form of placoid scales which are sometimes so numerous as to give the whole skin a rough surface forming shagreen.
The exoskeleton of the dogfish is mainly composed of placoid scales, each of which consists of a little bony base imbedded in the skin, bearing a small backwardly-directed spine formed of dentine capped with enamel.
A =placoid scale=[3] is developed on a papilla of the dermis which projects outwards and backwards, and is covered by the columnar Malpighian layer of the epidermis.
These facts seem to point to stages in the fusion of placoid bases, and their withdrawal from the surface to become incorporated with the cranial apparatus as membrane bones, a process entirely completed in the mammalian type.
Besides the teeth there perhaps remain relics of the placoid scales in the anatomy of the higher vertebrata, in the membrane bones.
This bone lies on the roof of the frog's mouth, and bears a number of denticles, and altogether there is a very strong resemblance in it to a number of placoid scales the bony bases of which have become confluent.
The first Vertebrata to appear in the fossil history of the world are fishes; fish spines and placoid scales (compare dog-fish) appear in the Ordovician rocks.
How placoid scales may have given rise to these structures will be understood by considering such a bone as the vomer of the frog.
The skin of the dog-fish is closely set with pointed tooth-like scales, the placoid scales, and these are continued over the lips into the mouth as teeth.
Similar bone-beds occur in the Carboniferous Limestone of Armagh, in Ireland, where they are made up chiefly of the teeth of fishes of the Placoid order, nearly all of them rolled as if drifted from a distance.
In both of these genera, the posterior concave face is armed with small spines, as in that of the fossil Hybodus (Figure 378), a placoid fish of the shark family found fossil at Lyme Regis.
The fishes of the Wealden are chiefly referable to the Ganoid and Placoid orders.
A single well defined placoid fossil in the Bala limestone as fully proves the existence of placoid fishes, during the period of its deposition, as if the rock were made up of placoid fossils, for it is not a question of numbers, but of rank.
The scales of sharks, dog-fish and rays are called placoid for they are toothed; not only so but their arrangement is frequently quite dissimilar to the scales of ordinary fish.
It bore no external markings, and the section exhibited but an indistinct fibrous structure, sufficient, however, to indicate its vegetable origin.
It consisted of a flattened cylinder, in some of the specimens exceeding a foot in length by an inch in breadth, and traversed on both the upper and under sides by a mesial groove extending to the extremities.
But one of the most remarkable weapons of the period was the sting of the Pleuracanthus, another great placoid of the age of gigantic fishes.
Certain of the placoid elements belonging to that part of the skin which gives rise to the lining of the stomodaeum have their spines enlarged or otherwise modified to form teeth.
In the Selachians the bony skeleton is restricted to the placoid elements.
The distinctions between cycloid and ctenoid scales, between placoid and ganoid fishes, are vague, and can hardly be maintained.
It seems highly probable that the more deeply seated osseous elements occurring in these as in the higher groups arose in the course of evolution by the spreading inwards of bony trabeculae from the bases of the placoid elements.
In the groups of fishes above the Selachians, the coating of placoid elements shows various modifications.
Each placoid element consists of a spine with a broadly expanded base embedded in the dermis.
Such typical placoid scales are well seen on any ordinary skate.
Morphologically they are probably to be looked on (like placoid elements) as local exaggerations of the basement membrane.
In all probability bony elements occupying similar positions and described by the same anatomical name have been evolved independently from the ancestral covering of placoid elements.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "placoid" in a variety of sentences. We hope that you will now be able to make sentences using this word.
