of the Pelvic Plcxux in Acanthias vulgaris. 5 



centrum we suddenly meet with a centrum over which there issues no 

 spinal nerve (cf. fig. 2, centrum 49). This centrum has been always 

 reckoned as the first half-vertebra. 



Of course it might be argued that in this particular instance 

 centra 48 and 49 constitute the first two half vertebrae, and not 

 centra 49 and 50 ; and similarly for all other cases. 



Judging from analogous cases, such as that of Amia calva (Gadow 6, 

 p. 202 and fig. 14), this latter view may possess rather more support. 

 Until the question is settled by the investigation of the innervation 

 of the myomeres in this region more cannot be said. The point of 

 importance here is that a uniform system of nomenclature has been 

 adopted for all the specimens examined. It may be mentioned that 

 the mere size of the centra does not constitute an accurate indication 

 of this point of junction. Though the first " halb-wirbel " centrum 

 is usually smaller than that of the last " ganz-wirbel," yet a glance at 

 fig. 2b will show that this is not always so. In this case centra 48 

 and 49 belong to the first half-vertebra in spite of the fact that 

 centrum 49 is at least as large as any centrum of the whole 

 vertebrae. 



As the collector nerve lies along the outer side of the vena 

 parietalis, injection of this with osmic acid rendered visible even the 

 minutest branches which went to form it. The post-girdle nerves 

 were also dissected with the help of the same reagent, which showed 

 them up very readily in the fresh material used. 



One hundred specimens in all were examined, of which fifty were 

 males and fifty females. Of the males twenty-nine were adults 

 ranging from 35 cm. to about 90 cm. in length, whilst forty of the 

 females ranged between these sizes. The rest were old embryos about 

 25 cm. long, very nearly ready to be born, which for all intents and 

 purposes, as will be seen below, could be reckoned as adults. The 

 tabulated results of the points examined are given in Table I. 



