A Cdndilmlioii to tlio AiKtttmiy of Siren iMccrtiiia. C55 



between the gills is not the occipital region, as it appears at first, 

 but the anterior portion of the dorsal muscular system, the division 

 of which into myotoms is plainly visible along the back and sides. 



The depression in the median line, immediately in front of the 

 dorsal muscles, lies between the two masseters, while two snuill pro- 

 minences somewhat anterior to these, and meeting in the median line, 

 signify the position of the temporales. 



Upon the ventral surface a fold of skin, extending transversely 

 across between the first pair of external gills, is formed by the posterior 

 border of M. intermaxillaris posterior, and may be taken as the posterior 

 limit of the head upon the ventral side. 



The lateral borders of the upper lip are prolonged into over- 

 hanging folds, Plicae labii superior is, which cover the angle of 

 the mouth and the greater portion of the lower lip. 



The lower lip also possesses a fold. Plica labii inferioris, 

 which is reflexed, thus forming a noticeable groove or wrinkle. 



The external nasal openings differ both in shape and position from 

 those of the other Perenuibranches and Derotremes, in that they are 

 long, narrow slits, obliquely placed and situated somewhat on the 

 ventral side of the lower lip, in such a position as not to be visible 

 from above. The openings are quite near the angle of the mouth 

 but this fact is somewhat disguised by the presence of Ihe labial folds. 



The eyes are very small and the ears are not visible externally. 

 The external gill openings are three in number, which is an important 

 specific distinction between the genuine Siren and the genus Pseudo- 

 hranchus, where there it but one. The external gill consists of the 

 basal piece or trunk, which bears two rows of branches. The best 

 developed of these, the first in the third gill-trunk, for example, bears 

 again a double row of smaller trunks or branchlets, and in the largest 

 of these a farther branching is often noticed. 



After the above examination of the external parts, the skin may 

 be removed from both dorsal and ventral surfaces of the head, but 

 should be left upon the external gills, the lips, and lip folds. 



The skin above the nasal capsules is intimately connected with 

 the subjacent parts and is best left until the special investigation of 

 the region. The integument is easily separable from the muscular 

 surfaces, but it must be cut away from the intermuscular ligaments 

 described below. The cornea of the eye, being only a differentiated 

 portion of the integument, is removed with it, leaving the Bulbus oculi 

 in its intermuscular position, protected only by ligaments and connective 

 tissue, but by no cranial parts. 



