MUSEUM OF COMPARATIVE ZOOLOGY, 3 



An open canal, the lateral line, extends on each side from the back of 

 the skull to the end of the tail. Other open canals, branches of the 

 same system, are seen beneath and on the side of the head (Plates I. and 

 IV.). On the skull the canals are covered and appear as lines of pores 

 (Plate III.). In addition to the main lines indicated by the artist, a 

 transverse branch extends to the post-orbital process, where it makes a 

 short backward turn, then descends on the side of the face to join a line 

 parallel with the mouth and extending forward from the angles of the 

 jaws. Behind the post-orbital process, between it and the spiracle, there 

 are short lines and groups of small pores. The line above the mouth con- 

 tinues to the tip of the snout ; in front of the eye a branch passes above 

 the nostril, and, a short distance in front of the latter, appears on the 

 upper surface and turns backward as the main branch. Smaller pores 

 are numerous over various parts of the head. 



From the bases of the pectorals to those of the ventrals is about 

 twenty-two inches. This section of the body is slightly compressed ; its 

 depth in life was probably in the neighborhood of four, and its width 

 somewhat near three inches. A prominent doubled or grooved keel along 

 the median line of the belly adds considerably to the depth. Toward 

 the pectoral arch and at the pelvis the keel loses its prominence ; it is 

 largest near the middle of the total length, where it projects three quar- 

 ters of an inch, and the groove has a depth of one third as much (Plate 

 XX., A, B). At first, the specimen being a fertile female, the promi- 

 nence of the keel and its folds was looked upon as a possible sexual 

 development, appearing while the young were carried. Study of the 

 structure and failure in a search for similar growths in other sharks 

 cause a change of opinion. From their position, shape, and extent, it is 

 evident the folds will furnish support to one of the theories of the origin 

 of paired fins. The muscle of the inside of the keel corresponds to the 

 rectus abdominis of other vertebrates. It differs somewhat from the other 

 muscle of the abdomen, as will be seen from the description given below. 



Situated behind the middle of the body, on the tapering portion, and 

 being large, the posterior fins give the specimen the appearance of being 

 more uniform in size from end to end than it really is. None of the 

 fins are at all rigid, but, on the contrary, all are very soft, and, like the 

 body itself, extremely flexible. They are covered with shagreen except 

 near the outer edges, which are very thin and membranous. There is a 

 single comparatively small dorsal. This fin begins above the origin of 

 the anal, as is indicated by the peculiar armature of the upper edge, and 

 gradually rises backward to terminate in an acute point about opposite 



