THE YPSII.OID APPARATUS OF UROUELES. 267 



6. Nccturns niaculatus. In Ncclitrits there is no vestige what- 

 ever of a Cartilago ypsiloidcs. The pubo-ischium is produced 

 anteriorly into a long median point (the epipubic process of 

 previous writers), the sides of which slant gradually posterio- 

 laterally to the outer angle of the pelvis where a lateral process 

 is slightly developed (Fig. 8). The first myotome of the rectus 

 abdominis muscle is correspondingly narrow at the ventral mid- 

 line, and much wider toward its lateral boundary. Its fibers are 

 inserted along the entire anterior margin of the pubo-ischium 

 and show a differentiation into a medial (rH) and a lateral portion 

 (ra), the latter inserted into the lateral portion of the pubis. In 

 some specimens this lateral portion is more distinct than in others, 

 and occasionally assumes the character of a semi-independent 

 muscle, as in Diemyctylus. There is no indication whatever of a 

 muscle dorsal to the main mass of the rectus abdominis and in- 

 serted into the linea alba, to suggest a vestigial ypsiloid muscu- 

 lature. The M. pubo-ischio-femoralis intcrmts moreover, has its 

 origin wholly from the pubo-ischium instead of arising in part 

 from the linea alba as it does in the case of the lungless sala- 

 manders. There is therefore absolutely no indication of the 

 previous existence of a Cartilago ypsiloidcs in this species. 



7. Amphiuma means. Ampliinma means shows a similar failure 

 of all trace of an ypsiloid apparatus. The pubo-ischium lacks a 

 mid-ventral symphysis in this form and there is only a slight an- 

 terior prolongation of the cartilage on either side of the mid-ventral 

 line where the two halves of the girdle are in contact. The 

 muscular differentiation in this region is, however, practically 

 similar to that already described for Nccturns. 



8. Siren laccrtina. - This species not only shows no trace of 

 a pelvic girdle and appendages, but, as might be expected, there 

 is also no muscular differentiation to indicate the former presence 

 of an ypsiloid apparatus. 



B. Development of the Yfisi/oiil Apparatus. 



My material for the study of the development of the ypsiloid 

 apparatus was somewhat limited. It consisted of (i) larvae of 

 Amblystoma opacum various stages from 37 to 50 mm. in 

 length ; (2) a series of horizontal sections of the larvse of Triton 



