28 PISCES. 



Sub-Class 1. Elasmobranchii. 



Fin-spines and dermal tubercles consisting of vaso-dentine 

 occur among almost the earliest known fish-remains, being met 

 with in the Upper Ludlow Bone-bed and in the corresponding 

 Upper Silurian rocks of the Island of Oesel in the Baltic Sea. 

 Presumably, therefore, the sub-class of Elasmobranchs dates at 

 least from Upper Silurian times, although no satisfactory 

 skeletons have been discovered in older formations than the 

 Lower Devonian. 



The earliest fin-spines, of which good examples are known 

 both from the Upper Ludlow Formation and from the Passage 

 Beds between this and the Old Red Sandstone, much resemble 

 the dorsal spines of the modern Cestracion and are provisionally 

 named Onchus. They are all bilaterally symmetrical, of small 

 size, not exceeding two or three inches in length, and having a 

 long base of insertion sharply marked off by an oblique line 

 from the exserted portion, which is ornamented with smooth or 

 faintly crenulated longitudinal ridges. The internal cavity of 

 the spine is open posteriorly towards the base, and there are no 

 denticles on the hinder edges. The tissue is characteristic 

 vaso-dentine, and a microscopical examination readily dis- 

 tinguishes the fossil from fragments of ribbed Crustacean 

 spines often met with in the same strata. 



The associated dermal tubercles are commonly named 

 Ccelolepidse (" hollow scales ") and have been described as 

 Ccelolepis and Thelodus or Thelolepis. Each is formed round a 

 single papilla, is hollow within, and coated with structureless 

 enamel (ganoine) on the surface which was originally exposed ; 

 this external layer, moreover, is separated by a constriction 

 from the base. The tubercle is in fact the simplest type of a 

 shagreen-granule, and it occurs in great numbers both in the 

 Ludlow Bone-bed and in the Oesel Limestone. 



A few minute Elasmobranch teeth also occur in Oesel 

 (Monopleurodus, Rhabdiodus), but they are very simple. They 

 all appear to exhibit a broad base, and the crown consists of 

 one or more conical cusps. 



