100 BULLETIN OF THE 



subrostrals, as in both of the preceding, but do not reach them by a compara- 

 tively simple course, as in D. dipterurus, or through complex areas, as in 

 D. nudus. At the start they run forward to turn sharply back and to the 

 side, before reaching the pleurals; then they commence a series of perplexing 

 and seemingly erratic turns, doublings, and zigzags, that ultimately bring them 

 opposite the second gill clefts and thence forward to the short orbito-nasals. 

 Equally crooked is the course of the subrostral ; it makes two prominent loops 

 at the side of the nostril, one in front of it, and another at its inner side upon 

 the nasal flap, before making its way directly to meet the rostral at the tip of 

 the rostrum. The nasals partake slightly of the tendency toward sinuosities, 

 as also the prenasals, orals, angulars, aud jugulars. The median is short. 



No attempt has been made in the drawing to follow the tubules of the head 

 or back. The laterals continue along the sides of the tail throughout the whole 

 of its great length. Under the snout the subrostral is difficult to trace, so 

 much so that the connection with the rostral may yet be considered an open 

 question. 



Pteroplatea. 



Plates 3XIII.-XI/V. 



Nowhere else in the order, so far as it has come under our notice, does the 

 development of the canal system attain such a degree as in this genus. So 

 great is the number of tubules and branchlets that the larger portion of the 

 upper surface is a tangle of minute vessels. They are most closely grouped in 

 a broad band along the anterior margin, and on the head ; posteriorly they are 

 not nearlv so much crowded. A space entirely unoccupied by them is found 

 on the middle of each pectoral, whence it extends upon the branchial area. 

 Smaller spaces, quite as free from them, appear in the scapular areas, and above 

 the abdomen behind the scapulars. On the ventral surface the extent of the 

 canals is not so remarkable; it is not much greater on this side of the disk than 

 in species of Dasybatus. The only cause that suggests itself for such an ex- 

 traordinary development of the system on forms that seem so poorly provided 

 with means of progression, of defence, or of procuring subsistence, is a greater 

 dependence on vibrations in the water for knowledge of the presence of ene- 

 mies or of prey. 



Pteroplatea hirundo (Plate XLIII. fig. 1). Resemblances in shape of disk 

 between this species and P. valenciennii are accompanied by similarities in the 

 figures outlined by the main canals. The pre-scapular area is large, and lies in 

 front of a scapular network, in the formation of which the post-scapular is also 

 concerned. Anteriorly the smaller canals are less numerous and much more 

 loosely arranged than in either of the following species. Posteriorly, also, the 

 vessels are less abundant, and their general appearance is more straggling and 

 scattered than is the case on the same locality in those forms. The pleural 

 appears on the lower surface about half-way from the median to the tip of 

 the snout. 



