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Show 1°90.] FIN-SKELETON OF BATOID FISHES. 681 numerous than are those of the Sharks, and that the would-be mesopterygium of the Trygon furnishes, like that of Raia, more than is demanded of it (cf. Table, and especially Trygon uarnak). FItr.9 Tig.10 Fig. 9. Horizontal section through the left pectoral fin-base, with its related girdle, in Trygon pastinaca, 2 • One half nat. size. Fig. 10. A similar section through corresponding parts of Torpedo narce, rf. Nat. size. References 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 pastinaca, 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 it is the subdivision of the so-called mesopterygium 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 individual 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- |
