1890.] ON THE PECTORAL FIN-SKELETON OF BATOID FISHES. 675 



i.l. Large intestine. 

 ' i.s'. Small intestine (Bursa Entiana or duodenal segment). 

 i.s". Small intestine (ileal or valve-bearing segment). 



Iff. Suspensory ligament of liver. 

 o.ff. Gastro-hepatic omentum. 

 om. Lesser omentum. 



po. Pancreas. 

 py. Stomach (pyloric sac). 



re. Left kidney. 

 s. Spleen. 



ts. Testis. 



v'. Pyloric valve. 



v". Intra-duodenal valve. 

 v'''. Spiral valve. 



5. Observations on the Pectoral Fin-Skeleton of the Living 

 Batoid Fishes and of the Extinct Genus Squaloraja, with 

 especial reference to the Affinities of the same. By 

 G. B. Howes, F.Z.S., F.L.S., Assist. Professor of 

 Zoology, R. College of Science, S. Kensington. 

 (From the Huxley Research Laboratory.) 

 [Eeceived December 2, 1890.] 



I. — The Pectoral Fin-Skeleton of the Trygonid Pteroplatea hirundo. 



The pectoral fin-skeleton of this fish is supported for the most 

 part upon large pro- and meta-pterygia {pp., mt., figs. 1 & 2), and 

 the first-named cartilage is in articulation with the shoulder-girdle 

 and the mesopterygium, one or both, by means of well-defined 

 synovial joints. Intercalated between the pro- and meta-pterygia 

 (pp.,mt.) are two well-defined cartilages {ms.,np.), each resulting from 

 the fusion of the bases of a number of parallel rays of the fin-axis. 

 These cartilages are plate-like and expanded in the manner of the 

 mesopterygium of the Selachoidei, and with that they might api^ear 

 at first sight to be jointly homologous. 



The mesopterygium of the Selachii is well known to be variable 

 in its degree of extension outwards, in proportion to which it forms 

 a more or less efficient support for the axis of the fin ; great as is 

 this variation, there is no known Selachoid fish in which the articular 

 base of the mesopterygium is furnished by more than four ravs. 

 In Pteroplatea some 18-23 or more of the 21-26 rays which 

 support the fin-axis are in direct apposition with the limb-girdle. 

 The detailed characters and variations in fusion of these are 

 sufficiently represented in the accompanying figures (figs. 1 & 2) ; 

 but, concerning their fundamental relationships, there are one or 

 two noteworthy features. In the younger of the two examples 

 (fig. 1) the anterior of the two supposed mesopterygia (tns.) is in 

 mere fibrous connection with the shoulder-girdle ; and the pro- 

 pterygium (pp.), which bears postero-internally a facet for synovial 

 articulation with the latter, furnishes a condyle for articulation 



