MUSEUM OF COMPARATIVE ZOOLOGY. 101 



Pteroplatea marmorata (Plate XLTII. fig. 2). Greater length of disk and 

 less lateral extension naturally bring about differences between this species 

 and the following, in regard to shapes and outlines of areas, etc. For instance, 

 the curve in the pleural behind the shoulder is comparatively deeper and 

 shorter, and the finger-like area, projected toward the outer angle, stretches 

 obliquely backward instead of nearly straight outward. 



A pair of strong pre-scapular branches are situated close together on the for- 

 ward portion of the scapular curve. These are not connected with the scapular 

 area, which is at some distance from them and close to the pleural canal. An 

 elongate post-scapular branch lies near to, and for some distance parallel with, 

 the pleural. The masses of tubules and branchlets are more compact than in 

 the preceding, and less dense than in the following. On the lower surface the 

 pleural emerges two fifths of the distance from the median to the tip. 



Pteroplatea valenciennii (Plates XLIV., XLV.). Neither of the other spe- 

 cies figured possesses so great a number of tubules as this one. Forward, the 

 tubes are completely hidden. The laterals may be traced without removing 

 the smaller vessels, except close to the back of the head. In the scapular sec- 

 tion the curves are extensive, though not very prominent toward the pectoral. 

 Near the aural the tubes are rather close together; from their bends in this 

 vicinity long tubules, with two or more branches, extend back beyond the 

 shoulders. Long tubules, also, put out from the sides of the laterals over 

 the abdominal chamber, beyond which the main canals continue to the end 

 of the tail. A couple of pre-scapular areas lie in front of the scapulars ; the latter 

 originate in a plexus of scapular and post-scapular branches, in which it is diffi- 

 cult to trace the main line. A broad shallow bend, toward the hinder margin, 

 brings the pleural behind and outside of the middle of the pectoral ; there it 

 turns forward and slightly inward till in front of the middle, where it turns 

 directly toward the outer angle, making a deep notch, open backward. Some 

 distance from the angle of the fin the tube turns toward the eye, nearly paral- 

 lel with the margin, and, after meeting four or five of the suborbital tubules, 

 descends near the forehead between the suborbital and the cranial. Anteri- 

 orly the tubules are numerous, branching into a thicket ; posteriorly they are 

 not so many or so short, and do not present such a confused mass of branch- 

 lets. Occipital branches occur on both laterals and occipitals. Opposite the 

 orbits the orbitals make a prominent bend outward. On this bend are the 

 tubules connecting with the pleurals; behind it are several long branches; and 

 in front of it are a number of long tubules reaching forward. The orbitals 

 descend in advance of the pleurals, and much nearer the rostrals. Ou the 

 cranial the ante-orbital curve is sharp and produced. 



. Ventrally the pleurals extend near the front edges of the pectorals, for more 

 than two thirds of the length of the latter, before turning toward the abdomen 

 and meeting the jugular extension a little backward from the scapular arch. 

 They emerge in the posterior third of the distance from the median to the tip 

 of the snout; not as shown in the sketch, where the median is too far back and 

 the pleural too far forward. The orbito-nasal is a mere point, as if the tubes 



