738 MR. C. TATE REGAN ON [June 19,. 



centrum. In Mustelus, accoixling to the same authoi-, the centra 

 become more and more elongated in the posterior prsecaudal region, 

 and we pass from normal vertebree to some in which the broader 

 neui-al plates bear an extra median cartilage, which are followed 

 by others with two extra median cartilages corresponding to the 

 still broader neural plates ; then follow the " half- vertebrae." In 

 Scyliorhimis, as described by Ridewood, the transitional stages 

 are (1) the broadening of the neural plate and introduction of an 

 extra median cartilage, (2) the division of the centrum and of the 

 neural and haemal plates, (3) the intercalation of an extra inter- 

 neural between two contiguous neurals. 



Hasse considered polyspondyly to be the original condition, but 

 it seems more probable that monospondyly is primitive and that 

 diplospondyly and polyspondyly ai-e secondary. Hidewood has 

 suggested that the diplospoiidyly of the caudal myotomes of the 

 Euselachii may be due to the need for greater flexibility in this 

 region, and if we add to this the fact that the caudal myotomes 

 are longer than those of the trunk *, we seem to have a basis for 

 a possible explanation of this phenomenon. 



Some explanation of the structure of the mixopterygia (text- 

 fig. 117) is necessary. These paired intromittent organs, which 

 have been especially studied by Jungersen and Huber, are append- 

 ages of the pelvic fins, and are supported by a cartilaginous skeleton. 

 A groove extending for the whole length of each appendage is the 

 duct of a glandular sac at its base, which may or may not contain 

 a special glandvilar body. In the Euselachii the skeleton of the 

 appendage consists principally of an axial cartilage, either a pro- 

 longation of the basipteiygium, or more probably the last radial,, 

 enlarged and modified. Proximally one to four short segments 

 of the axial cartilage may be defined, and an accessory cartilage^ 

 which appears to be the displaced penultimate radial, is attached to 

 these or to the axial cartilage proper. The groove of the appendage 

 is bordered by a dorsal and a ventral marginal cartilage, which 

 are united basally to the axial cartilage. Articulated to these 

 distally are dorsal and ventral terminal cartilages, and a varying 

 number of additional terminal pieces may be present, some of 

 which may be spinous. Finally, one or more covering pieces, 

 sheathing the terminals, may be present. 



The contents of the glandvilar sac are driven by means of 

 special muscles along the duct formed by the closing of 

 the edges of the groove, and out through the aperture formed by 

 the extension of the terminal pieces, which appear also to have 

 the function, when extended, of retaining the organ in position,. 

 From the structure of the mixopterygia alone Huber has arranged 

 the Euselachii in three groups— the first corresponding to the 



* In a specimen of Sqimlns acantJdas I find 4 anterior praecaudal vertebrse have- 

 the same length as 3 posterior prtecaudals, 5 posterior pra3caudals are equal in length 

 to 8 anterior caudal " half-vertebrae," which again are as long as 6 " half-vertebrae " 

 of the region behind the second dorsal fin ; one of these last, in fact, is of the same 

 size as one of the anterior trunk vertebrae, showing that the myotome is twice as long.. 



