1890.1 



FIN-SKKLETON OF BATOID FISHES. 



681 



numerous than are those of the Sharks, and that the would-be 

 mesopterygiurn of the Trygon furnishes, like that of Raia, more than 

 is demanded of it {cf. Table, and especially Trygon uarnak). 



ms. 



//Z/7, 



Fig-, 9 



Fig. JO 



Fig, 9. Horizontal section through the left pectoral fin-base, with its related 

 girdle, in Trygon fostmaca, J , One half uat. size. 



Fig. 10. A similar section thi-ough corresponding parts of Torpedo narce, (f . 

 Nat. size. 



Eeferences and other details as for figs. 1-8. 



In one of my specimens the basal cartilages are in an altogether 

 exceptional and highly interesting condition. The case referred to 

 is that of an adult female of Trygon imstiiiaca, by far the largest 

 of the individuals examined (see Table). The supposed mesopterygial 

 rays instead of being from 12 to 13 in number, reached, in it, 

 the total of 19; and of these 8-9 were in articulation with the 

 shoulder-girdle, instead of from 3-5. The base of one of these fins is 

 represented in fig. 9, and that which is most remarkable concerning 

 ic is the subdivision of the so-called mesopterygiurn into two plates 

 {ms., np.), each independent of the other, and both in synovial 

 articulation with the middle glenoid facet. Both fins were similarly 

 modified, except for the fact that whereas on the right side the 

 demarcation line between the two plates lay between rays 9 and 10, 

 on the left it lay between those numbering 11 and 12. 



Comparison of this pair of fins (fig. 9) with those of Pteroplatea 

 figs. 1 & 2) reveals a striking similarity in structure, and it must 

 be admitted that the characters of the basal cartilages of the in- 

 dividual Trygon pastinaca in question depart from those of its 

 species, as hitherto described, exactly as they approximate towards 

 those of Pteroplatea. In other words, what then is the nature of 

 the relationship between these two? as we have once more to face 

 the correlation of the existence of an apparently duplicated meso- 



