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Bashford Dean Memorial Volume 



(palatoquadrate) has a preorbital articulation with the cranium, while in the Hybodontidae 

 the attachment is postorbital. Nine genera, including Paleospinax and Synechodus, were 

 assigned to the Hybodontidae, leaving one genus, Cestracion, for the Cestraciontidae. 



In the second German edition of his "Grund^iige der Palaeontologie", Zittel (1911) 

 listed in his family Cestraciontidae seven genera including Cestracion. Eight other genera, 

 including Hyhodus and Orodus, made up his family Hybodontidae. The most recent 

 (fourth) German edition of Zittel (1923) departs only slightly from this classification. In 

 the separation of the two families, Woodward appears to have taken part. In the second 

 English edition of Zittel, revised by Woodward in 1932, the family Cestraciontidae 

 includes only three genera {Cestracion, Paleospinax, and Synechodus) while the family 

 Hybodontidae comprises thirteen genera including Hyhodus and Orodus. Woodward's 

 definitions of the two families deserve careful attention : 



DISTINCTIVE CHARACTERS OF THE HYBODONTIDAE AND THE CESTRACIONTIDAE 



According to Woodward in Zittel (1932). 



HYBODONTIDAE 



Teeth numerous, mostly obtuse, never 

 fused into continuous plates; several series 

 simultaneously in function. Notochord 

 persistent. Some ribs long and slender; 

 neural arches also long and slender. Each 

 of the two dorsal fins armed with a spine, 

 which is as deep as the fin; the spine orna' 

 mented on the sides and bearing one or two 

 rows of posterior denticles. Anal fin with- 

 out spine. Tail heterocercal. Paired 

 hooked head spines often present. Devonian 

 or Lower Carboniferous to Cretaceous. 



CESTRACIONTIDAE 



Teeth as in Hybodontidae. Vertebral 

 centra cyclospondyHc or asterospondylic. 

 Ribs and neural arches very short and broad. 

 Each of the two dorsal fins armed with 

 a spine which is less deep than the fin; the 

 spine is almost or completely unornamented, 

 and without posterior denticles. Anal fin 

 without spine. Tail heterocercal. No head 

 spines. Lower Jurassic to Recent. 



Several of the characters Hsted above are much alike in the two families. The degree 

 of this likeness, and its significance, need some evaluation; but first let us note some pos' 

 sible additions to the list of resemblances. Certain peculiarities in the form of the head 

 and anterior part of the body of some Cestracion ts, leading to the common name "Bull- 

 head Sharks", find a counterpart in fossil forms like Hyhodus (Text-figures 27 and 28). 

 This matter has been discussed on pages 660 and 686. A considerable degree of flatness of 

 the ventral surfaces of both head and body may also be common to the two families. 

 Woodward (1921) states that in their general appearance the Hybodonts resemble the 

 Cestracionts. The pectoral girdles of both Heterodojitus (Daniel, 1915, Fig. 8, pi. IV) 

 and Hyhodus (my Text-figures 27 and 28) are very strong. 



Some of the characters common to the two families are included in the definitions 

 presumably for comparison with other families in the same suborder, or to show inclusion 



