DEVONIAN FISHES OF IOWA 97 



from other fishes of no mean size to mollusks, crustaceans and 

 other invertebrates, or even to plankton. In their breeding 

 habits the sharks and Dog-fishes present many interesting fea- 

 tures. Unlike the generality of fishes, the eggs are fertilized 

 internally as a sequel to the copulation of the sexes. For this 

 purpose the males are furnished with special intromittent organs, 

 the nryxopterygia or so-called claspers, which are developed as 

 modifications of the hinder portions of the pelvic fins. Each 

 clasper is supported by an internal skeleton, consisting of several 

 cartilages derived from the radialia of the fins, and is traversed 

 along its inner aspect by a groove. "When sexual congress takes 

 place the claspers are thrust through the cloaca of the female 

 into the oviductal orifices, and in some instances it is probable 

 that they are retained in this position by hook-like denticles 

 developed at their free extremities. The seminal fluid then 

 flows along these conduits into the oviducts, in the upper por- 

 tions of which it meets and impregnates the eggs. After fertili- 

 zation the egg is enclosed in a dark brown horny egg-case, 

 secreted by the oviductal gland. 



Order PLEUROPTERYGII. 



Xotochord persistent; neural and haemal arches slender. 

 Paired fins with basalia and radialia arranged much as in the me- 

 dian fins of recent fishes. Claspers not observed. Apparently a 

 cutaneous flap much like an operculum covered the first of the gill 

 slits, which were at least five in number, and perhaps seven. 

 Jaws apparently hyostylic ; lateral line an open groove. 



The only known representative of this order, and at the same 

 time, as indicated by the position of its paired fins and other 

 features, the most primitive type of Elasmobranch yet discov- 

 ered, is the Upper Devonian and Lower Carboniferous genus 

 Cladoselache. Among all fossil fishes this genus is regarded as 

 furnishing the most important testimony in favor of the lateral 

 fin-fold hypothesis. In view of its extreme importance from a 

 morphological and phylogenetic standpoint, it seems desirable to 

 offer the following account of its organization, compactly drawn 

 up by Smith Woodward : 



"The fish (text-fig. 6, aide, p. 61) is elongated and round- 

 bodied, with a short blunt snout, and f orwardly placed eye. The 

 precise characters of the cranium are unknown; but the olfac- 

 tory capsules are large, placed close together, and terminal. The 



