32 



I'llOCKEDINGS OF XUE 



bevond the fin itself by a row of broad (juadraii^ailar cartilages. 

 These may also have been retained vvitliin tbt; l)odv-\vall. a's a 

 remnant of the support of a formerly extended fin-fojd ; or' they 

 may have begun to project outside the body-wall, as Jaekel repre- 

 sents them, to form the beginning,' of the p'addle-shaped fin whic-li 

 we lind in the next grade ol sharks, the Ichthyotomi. 1 think 

 that if they had already begun to enter into a movable paddle, 

 they would have been sudiciently well calcified to be preserved in 

 many specimens. 



We have already noted that the dorsal fin of Cladosdache is 

 subdivided without any essential modification of its primitive 

 stiffening rods of cartilage. It is interesting now to add that a 

 closely related genus, CtenacantJms, equally generalised, ha.s a 

 spine, of the ordinary shark-pattern, in front of each dor.-al iin. 

 Dr. Dean * has described part of a well-preserved specimen from 

 Ohio, and I have lately found that the nearlv complete fish 

 from the Lower Carboniferous of Scotland, nanied Ctenacanthus 

 cosieUatus byTraquairt, is not a Cestraciont, as hitherto supposed, 

 but a true Pleuropterygian. In the latter specimen (fig. 2), the 

 simple tai)ering, parallel rods of cartilage strengthening the 

 membrane are seen in both the pectoral and pelvic fins, and they 

 are clearly supported by a corresponding series of parallel rods 



Vicr. 2. 



Explanatory outline of the parts shown in the I 

 from llie Lower Carboniferous of Scotland, 



of anal Iin, displaced; c. caudal fin ; d. dorst 



fin ; p/v. pelvic fin : s. cartilages of pectoral arch. 



fossil Cienacauthus costellatKg 



, one-eigiiti) nat. size. a. part 



anal Iin, displaced; c. caudal fin ; d. dorsal fin with spine; pet. pectoral 



within the body-wall. Each dorsal fin-spine is supported by a 

 triangular plate of cartilage between the muscles ; but the mem- 

 brane of each dorsal fin is strengthened by the usual simple 

 tapering parallel rods, of which the foremost is much the largest 

 and stoutest. There are no traces of an ordinary anal fin, but°the 

 area of shagreen-covered skin already noticed by Traquair as 

 displaced beneath the caudal pedicle, probably represents the 

 horizontal lateral dermal expansion which is so peculiar a feature 



* Loc. cit. (1909), p. 249, pi. xxxiii. text-figs. 42-45. 

 t Geol. Mag. [3] vol. i. (1884), p. 3, pi. 2. 



