680 PROF. G. B. HOWES ON THE PECTORAL [DeC. 2, 



genus Raia {cf. Table), and of the variations with age in the articular 

 surfaces oiPteroplatea already alluded to (^ante p, 6/7, figs. 1 & 2). 

 The post-mesopterygial plate of the Trygouid and the free inter- 

 calary rays of the i?a?jW<s having been now proved to be homologous, 

 the question arises, which of them is to be regarded as the more 

 primitive representative of the other 1 That the Batoid type of fin 

 has been derived from a shorter Selachoid one by forward rotation 

 and general enlargement is sufficiently clear, from known facts of 

 development ; and, on comparison of the two types, it might at first 

 sight appear that the post-mesopterygial plate and free rays named 

 above both represent, together with the mesopterygium, the meso- 

 pterygium of the Selachoidei, and that the free rays of the Raiidce 

 may have arisen by dismemberment and segmentation of the pos- 

 terior half of that. Such a possibility is, however, irreconcilable 

 with the fact, already demonstrated, that all the distinguishing 

 features of the mesopterygium of the Selachoidei are realized by that 

 of the JRaiidce, apart from the intercalary rays. The last named are 

 related to Parker's "glenoid commissure" (cf. ante, p. 676), and 

 Gegenbaur has sought to correlate (/. c. p. 144) their origin with 

 what he terms the " stretching of the articular region " of the 

 shoulder-girdle. Be their original significance what it may, the 

 facts above described show them to be at present active in the pro- 

 duction of a fourth basal cartilage, phylogenetically the youngest of 

 the series. The now well-known fact already cited (p. 679) that the 

 basal pterygia of Gegenbaur arise by fusion of the bases of parallel 

 and originally distinct rays, shows the cartilage in question to be 

 serially homologous with the former. I have thus far alluded to it 

 as the post-mesopterygial plate ; as it can no longer be referred to the 

 mesopterygium, I propose to term it, as is consistent with its mode 

 of origin and with Gegenbaur's expressive nomenclature, the neo- 

 pterygium. 



III. — The Pectoral Fin-Skeleton of Trygon, TJrolophus, and Mylio- 

 batis, compared with that of 'R&ia. and Pteroplatea. 



The pectoral fin-skeleton of Trygon pastinaca has been already 

 described by Gegenbaur (/. c. p. 144) and Haswell (/. c. p. 35). 

 Both observers agree in regarding that basal cartilage which oc- 

 cupies " all the interval between the propterygiun and the meta- 

 pterygium " as the mesopterygium. Gegenbaur figures in relation 

 to it 13 rays of the axis, five of which reached the pectoral girdle. 

 I have dissected, in all, five individuals of this species, three of 

 Triton uarnah, and two of Urolophiis testaceus ; and in all but three 

 of the series the number of these rays exceeds that of Gegenbaur's 

 specimen. In all of them the mesopterygium is comparatively 

 short and plate-like. Both Gegenbaur and Haswell regard it as 

 the homologue of the Selachoid mesopterygium. If, however, the 

 two things be compared under the conditions which I have laid down 

 {ante, p. 679) in dealing with lihina and Raia, it will be seen that 

 the supposed mesopterygial rays of the Trygons are much more 



