1906.] 



SELACHIAN FISHES. 



753 



pectoral arch, Mixopterygia with two proximal axial segments, 

 with the marginal cartilages rather short and distal in position, 

 with three terminal pieces and a large ventral covering piece, and 

 with the glandular body extending nearly to the distal end of 

 the appendage. Yertebral centra with secondary calcifications 

 deposited either continuously or as concentric laminse, forming 

 ridges which show various patterns in cross section*. 



Text-fig. 122. 



Anterior part of cranium of Rhynchohatus (A), Torpedo (B), and Dasyiatis (C). 

 (A and B seen from below, after Gegenbaur ; C seen from above, after Haswell.) 



r, rostrum ; n, nasal capsules j p, praeorbital cartilages. 



The head and trunk, with the pectoral fins, form a smooth 

 subcircular disc, and the tail is rather short and stout, with a 

 longitudinal fold on each side. 



Division 2. Batoidei. 



No electric organs between the pectoral fins and the head. 

 Rosti'um, if developed, simple, unpaired. Preeorbital cartilages 

 simple, short or of moderate length, not extending forwards, 

 attached on each side to the lateral or posterior wall of the nasal 

 capsule (text-fig. 122, A & 0). Suprascapulae united to the ver- 

 tebral column t (text-fig. 123, C, p. 754). 



* The forms figured by Hasse in some cases bear a considerable resemblance to 

 the Lamnidse, in others to the Cestraciontidae. 



f According to Gegenbaur, who has been followed in this matter by other 

 anatomists, there is no separate suprascapula in these fishes, but a cartilaginous 

 expansion of the fused neural arches, to which the pectoral arch is on each side either 

 simply attached (Rhinobatidte, Raiida;) or articulated by means of a ball-and-socket 

 joint (Dasybatidae). 



Dissection of several genera has convinced me that this view is erroneous. The 

 neural arches in this region form a ridge to which the suprascapular cartilages are 

 firmly united, the line of junction in all cases remaining quite visible. Although 

 different enough from the Narcobatoid type, the distinction is not so marked as has 

 been supposed. 



