^24 MR. U. I. POCOCK ON ETHIOPIAN SPIDERS. [Juue 1 5, 



tudinal ridge on the lateral surface of the basal or proximal portion 

 of an inferior vertebral arch or " basi-ventral " cartilage (bv.). 

 From its point of attachment the rib is directed obliquely back- 

 wards and a little outwards, slightly overlapping the succeeding 

 " interveutral " or haemal intercalary cartilage (iv.), and extending 

 into the ventral edge of one of the fibrous septa separating two 

 successive myotomes of the body-wall. Hence it follows that in 

 the latter part of their course the ribs are situated immediately 

 external to the peritoneal lining of the subjacent coelomic cavity, 

 as in other Ganoids and in Teleosts. No trace of ossification could 

 be detected in any of the ribs. 



In two or three instances the cartilage of a rib was broken up 

 into two or more separate nodules, as if undergoing fragmentation 

 as a preliminary to suppression. 



Polyodon therefore possesses a series of distinct but fully deve- 

 loped and wholly cartilaginous ribs, in substantial agreement with 

 the account originally given by Staunius, but apparently over- 

 looked by every subsequent writer. 



When Polyodon is compared with its nearest living ally, Acipenser, 

 the differences in the relative development of their costal elements 

 are very striking. In the latter Ganoid, as is well known, nearly 

 all the pre-cloacal " basi-ventrals " possess ribs, comparable both in 

 size and in their relations to the coelomic cavity to the normally 

 developed ribs of other Ganoids and most Teleosts, and of these 

 the majority are more or less well ossified, only a few of the more 

 diminutive posterior ones being reduced to the condition of simple 

 cartilaginous rods or filaments. In Polyodon, on the contrary, the 

 ribs are restricted to about one-half of the normal costiferous 

 region of the vertebral column, and to this may be added their 

 relatively minute size and wholly cartilaginous condition. 



How far it is permissible to regard the ribs of Polyodon as 

 incipient, nascent, or rudimentary elements, or as degenerate and 

 vestigial structures, is by no means easy to determine with certainty, 

 but their obvious uselessness, and especially their occasional ten- 

 dency to undergo fragmentation, strongly suggest the probabihty 

 of the latter alternative. 



2. On the Spiders of the Suborder Mygalomorphse from the 

 Ethiopian Region contained in the Collection of the 

 British Museum. By R. I. Pocock, of the British 

 Museum (Nat. Hist.) . 



[Received June 2, 1897.] 



(Plates XLI.-XLni.) 



This paper, based upon the material contained in the collection 

 of the British Museum, deals with those species which are usually 

 known as Trap-door Spiders and with the larger kinds of hairy 



