LINNEAN SOCIETY OP LONDON. 33 



of Clacloselache. A row of broad cartilages among the obscure 

 remains of the upper lobe, and the pointed ends of the cartilaginous 

 rods in the lower lobe of the heterocercal caudal fin, are also sug- 

 gestive of the corresponding structures in the tail of Clacloselache. 

 The powerful heterocercal caudal fin of CladosekicJie and Ctena- 

 cantJiKs shows that these fishes were hahitually free swimmers, not 

 merely grovellers on the sea-bottom. The horizontally extended 

 shape of the paired fins of the Pleuropterygii, therefore, cannot 

 be explained as an adaptation to a life like that of the skates. 

 The fins can indeed only be regarded as the little-altered remnants 

 of primitive continuous folds, as already observed; and if the 

 axial skeleton were sufficiently well calcified to he preserved in 

 the fossils, the cartilaginous fin-supports would doubtless prove 

 to be correlated in number with the vertebral arches. 



ICHTnTOTOMI. 



The correlation just mentioned, which is postulated by our 

 present theory of the primitive vertebrate, is actually seen in 

 several well-preserved skeletons of the Carboniferous and Permian 

 sharks of the order Ichthvotomi *. These were for long the most 

 primitive Elasmobranchs known, but they exhibit an advance on 

 the Pleuropterygii and approach the ordinary modern Selachii 

 in the structure of the paired fins. The pectoral is a unibasal 

 " crossopterygian " fin, which may easily have been dex'ived, by 

 concentration of the carti]agiiu)us supports, from the polyhasal 

 Devonian type of fin, and I still think it may have passed into 

 the dibasal or tribasal fin which is specially characteristic of the 

 Selachians of later periods. The pelvic fin of the male bears the 

 typical Selachian clasper. The only known Ichthyotomi (e, g., 

 Pleura canth us, fig. 1 b) are elongated fishes with a diphycercal tail, 

 evidently bottom-dwellers, and the paddle-shaped pectoral fiu was 

 probably used as in the Dipnoan Ccratodus. 



Like the Pleuropterygii, all the Ichthyotomi hitherto discovered 

 have a persistent notochord without even the beginning of verte- 

 bral bodies. llasse's determination t of ring-vertebra3 in tv\o 

 specimens in the British Museum is based on mistaken observa- 

 tions : in the one case (no. 35015) the supposed vertebne are the 

 segments of the axis of a displaced pelvic fin, in the other case 

 (no. 19665) the structures ap[)ear to be the bases of neural arches. 



Din-ing recent years little has been added to our knowledge of 

 the Ichthyotomi. I would, however, remark that a new chondro- 

 cranium from the Upper Devonian of Gi-ermany which may belong 

 to a primitive shark Xt seems to show symmetrical clefts in its 



* See especially A. Friieuh, Faiiua der Gaskohle Permforni. Bohiiiens, 

 vol. iii. pt. i. (189U). 



t C. Hasse, Neiies Julirb. fur Min. etc. 1883, vol. ii. p. 65. 



I Jnfim-ina pandora. O. Jaekel. Palaeontol. Zeitschr. vol. iii. (1921), p. 217, 

 text-fig.s. 1-3. 



LINN, SOC. PROCEEDINGS. — SESSION 1920-1921. d 



