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Show 724 MR. R. I. POCOCK ON ETHIOPIAN SPIDERS. [June 15, 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 backwards and a little outwards, slightly overlapping the succeeding " interventral" 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 tbe 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 developed and wholly cartilaginous ribs, in substantial agreement with the account originally given by Stannius, but apparently overlooked by every subsequent writer. W h e n 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 aud in their relations to the ccelomic 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. H o w 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 tendency to undergo fragmentation, strongly suggest the probability of the latter alternative. 2. On the Spiders of the Suborder Mygalomorphge from the Ethiopian Region contained in the Collection of the British M u s e u m . By R. I. P O C O C K , of the British M u s e u m (Nat. Hist.). [Received June 2, 1897.] (Plates XLI.-XLIIL) 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 |