516 The Class Elasmobranchii or Shark-like Fishes 



in their fibrous structure." Some of the species have certainly 

 no teeth at all. The tail is always heterocercal, or bent upward 

 at tip as in the Cladoselache, not diphycercal, tapering and 

 horizontal as in the Ichthyotomi. 



The lower Acanthodeans, according to Woodward, "are the 

 only vertebrates in which there are any structures in the adult 

 apart from the two pairs of fins which may be plausibly in- 

 terpreted as remnants of once continuous lateral folds. ' In 

 Climatius, one of the most primitive genera (see Fig. 305), there 

 exists, according to Woodward, and as first noticed by Cope, 

 between the pectoral and pelvic (or ventral) fins a close and 

 regular series of paired spines, in every respect identical with 

 those supporting the appendages that presumably correspond 

 to the two pairs of fins in the higher genera. They may even 

 have supported fin membranes, though specimens sufficiently 

 well preserved to determine this point have not yet been dis- 

 covered. However, it is evident that dermal calcifications 

 attained a greate development in the Acanthodei than in any of 

 the more typical Elasmobranchs, and we may look for much 

 additional information on the subject when the great fishes 

 to which the undetermined Ichthyodorulites pertained became 

 known." (See Fig. 305.) 



The Acanthodei constitute three families. In the Acan- 

 hoessidcB there is but one short dorsal fin opposite the ana 1 , 

 and clavicular bones are absent. The gill-openings being pro- 

 vided with "frills" or collar-like margins, perhaps resembled 

 those of the living genus Chlamydoselachus, the frilled shark. The 

 pectoral spine is very strong, and about the eye is a ring of 

 four plates. The body is elongate, tapering, and compressed. 

 Acanthoessus of Agassiz, the name later changed by its author to 

 Acanthodes, is the principal genus, found in the Devonian and 

 Carboniferous. 



The species of Acanthoessus are all small fishes rarely more 

 than a foot long, with very small teeth or none, and with the 

 skin well armed with a coat-of-mail. Acanthoessus bronni is 

 the one longest known. In the earliest species known, from 

 the Devonian, the ventral fins are almost as large as the pec- 

 torals and nearly midway between pectorals and anal. In 

 the later species the pectoral fins become gradually larger 



