PLAGIOSTOMI 99 



form, supports a reference of the Conodonts rather to some 

 soft invertebrate genus. Certain parts of small Crustacea — 

 e.g., the pygiclium or tail of some minute Entomostraca — 

 resemble in shape the more simple Conodonts ; but when we 

 perceive that these bodies occur in thousands, detached, with 

 entire bases, and that any part of the carapace, or shell of an 

 Entomostracan or other Crustacean, has been rarely detected 

 in the lower Silurian Conodont beds, it is highly improbable 

 that they can have belonged to an organism protected by a 

 substance as susceptible of preservation as their own substance. 

 Much more likely is it that the body to which the minute 

 hooklets were attached was as soluble and perishable as the 

 soft pulp upon which the Conodont was sheathed. The writer 

 finds no form of spine, denticle, or hooklet in any Echinoclerm, 

 and especially in any soft-bodied one, to match the Conodonts ; 

 and concludes that they have most analogy with the spines, 

 or hooklets, or denticles of naked Mollusks or Annelides. The 

 formal publication of these minute ambiguous bodies of the 

 oldest fossiliferous rocks, as proved evidences of fishes, is much 

 to be deprecated. 



Class I.— PISCES. 

 Order 1. — Plagiostomi. 

 {Sharks, Rays.) 

 Char. — Endo-skeleton cartilaginous or partially ossified ; exo- 

 skeleton placoid ; gills fixed with five or more gill- 

 apertures ; no swim-bladder ; scapular arch detached 

 from the head ; ventrals abdominal ; intestine with 

 spiral valve.* 



The earliest good evidence which has been obtained of a 

 vertebrate animal in the earth's crust is a spine, of the nature 



* For an explanation of the technical terms in these characters, see the 

 article Ichthyology, especially of the scales, Enc. Brit. vol. xii., p. 216. 



