342 Bashford Dean Tvlemorial Volume 



chus, now existing but exemplifying the Cretaceous and Tertiary type) selected to 

 illustrate the types prevailing in successive periods of time, Woodward (1921) says: 

 "Very soon the remnants of lateral fin folds, which must have acted merely as two pairs 

 of balancers in these fishes [the earliest known fossil elasmobranchs] concentrated into 

 paddles, and these again passed into stout-based fins adapted for swimming." It is not 

 explicitly stated, by the author quoted, that he regards this succession of types of paired 

 fins as a phylogenetic series, but one may infer that he considers the breadth of base of 

 the paired fins of Hyhodus and Chlamydoselachus as something secondarily acquired. 



It is known that Dean was an ardent advocate of the fin-fold theory for which he 

 (1894 and 1895) obtained interesting evidence in the case of the fossil Cladoselache 

 (Text-figure 5). The question of the origin of paired fins was one of the problems Dean 

 had in mind while he was searching in Japanese waters for embryos of Chlamydoselachus, 

 Cestracion (Heterodoiitus) and other primitive fishes. Subsequently, Dean's material 

 was studied by Osburn (1906 and 1907) who defended the fin-fold theory against 

 the attacks of those who favored the opposing gill-arch theory originally proposed by 

 Gegenbaur (1865). 



ABDOMINAL OR TROPEIC FOLDS 



The abdominal or tropeic folds are a pair of slender longitudinal thickenings of the 

 ventral abdominal wall, situated close to the median line and separated by an external 

 groove. They are figured and comprehensively described by Gudger and Smith (1933, pp. 

 283-284, Text-fig. 12), and are shown in transverse section in various figures inserted 

 in my chapter on the muscular system (p. 381). 



No satisfactory explanation has ever been advanced to account for the presence of 

 the tropeic folds, which are structures pecuHar to Chlamydoselachus. Concerning them 

 Carman (1885.2, p. 3) wrote: "From their position, shape and extent, it is evident that 

 the folds will furnish support to one of the theories regarding the origin of paired fins." 

 I agree with Braus (1898) that "Der Kiel des Chlamydoselachus hat zur Cenese der 

 paarigen Gliedmassen nicht die geringste Beziehung." In my section on the muscular 

 system there is given a fairly satisfactory explanation (illustrated by Text-figure 58, p. 386) 

 as to the manner of embryonic development, but this does not answer the question as to 

 the fitness of these peculiar structures for the needs of Chlamydoselachus in its particular 

 environment. One can infer from their form and position that they may have some sHght 

 utility in locomotion similar to that afforded by the keel of a ship : but in some specimens 

 they are too small to be of any appreciable use in this way. 



SCALES AND TEETH 



The variations in the form of the placoid scales or dermal denticles of Chlamy- 

 doselachus on different parts of the body, the form of the teeth, and the arrangement of 

 the teeth in rows have been described by Carman (1885.2), Rose (1895), and by Gudger 



