1-40 Transactions of the Society. 



each side. The tail seems to have remained flexible. The plates- 

 consist of three layers, an inner " nacreous " layer of lamellae, a 

 thick middle one of polygonal cancellse, and an outer hard layer of 

 vasodentine. 



The Ostracodermi comprise the Cyathaspis, Ptcraspis, Cephal- 

 aspis, Pterichthys, Bothriolepis, and some others. The bodies of 

 Pteraspis, Cephalaspis and Pterichthys, were covered with dermal 

 enamelled scales or plates. 



They mimic in a singular manner the contemporary giant 

 Crustaceans, the Eukypterida. 



Quite lately, a number of new forms have been discovered in 

 the Upper Silurian of Lanarkshire, and described by Dr. E. H. 

 Traquair. They are considered to be primitive Heterostraci, or 

 Pteraspidian fishes, covered with a dermal armour of shagreen-like 

 granules (LanarJcia spinosa), or an outline of plates with a central 

 shield and a series of polygonal plates (Drepanaspis).* 



The Ostracodermi, as a group of early vertebrates, are quite 

 extinct ; and their range in past time seems to have been very 

 limited, namely, from the Upper Silurian to the Upper Devonian. 



If the Marsipobranchii of to-day (the " Lampreys " and the 

 " Hag-fishes ") were really represented by the Cambrian Conodonts 

 and the Old Eed Sandstone Palmospondylus, then these lowly 

 vertebrates may claim as great a range in time as any of the 

 Invertebrata ; but this point is not as yet definitely established. 



Pisces. — We have spoken of the preceding groups as Fishes, 

 but they lack the important character of possessing a lower jaw, 

 they also have only a notochordal skeleton, and they do not always 

 possess paired appendages. 



True Fishes begin with the class Elasmobranchii, the most 

 ancient of which are the sharks, which extend as far back in time 

 as the Lower Devonian, the entire skeleton is cartilaginous, only 

 the teeth and the periphery of the vertebras being calcified ; but 

 in many species the primitive notochord is persistent ; the gills are 

 not covered by an operculum, but are pouch-like, having distinct 

 clefts on each side ; they have both median and paired fins, and 

 the tail is heterocercal. 



Examples of Early Sharks have been discovered in a remarkable 

 state of preservation in the Upper Devonian of Ohio, showing the 

 complete outline of the fish with its fins and tail preserved. 

 Although the jaws are cartilaginous the teeth in all are coated 

 thickly with enamel and are well preserved. They also possess 

 bony and enamelled dorsal spines, and microscopic shagreen 

 dermal ossicles in the skin. This type is very persistent, and its 

 remains are met with in almost every formation from the Devonian 

 to the seas of the present clay. 



* Gcol. Mng, 1902, pp.289-291. 



