Vol. 70.] VERTEBRATES FROM BRITISH EAST AFRICA. 183 



The plastron (fig. 3, B) is incomplete, the anterior and posterior 

 ends being broken away. The characteristic lateral mesoplastra 

 (Ms.p.) are well preserved. The left xiphiplastral {Xi.p.) has 

 come away from the matrix, and its impression shows that its 

 upper surface bore a roughened facet for union with the pubis, 

 as usual in the group. 



No living species of this genus is now found in Africa, but several 

 have been described from the Upper and Middle Eocene of the 

 Eaymn (such as JPodocne mis antiqua, P.fajumensis, P. stromeri 1 ) 

 and from the Lower Miocene of Moghara (P. cegyptiaca^). The 

 specimen here described resembles the type of P. cdgyptiaca in 

 many respects, and, moreover, is from a bed of the same age. For 

 the present, therefore, it will be best to regard it as a young 

 individual of that species. Both specimens are very similar to 

 individuals of P. maclagascariensis of corresponding ages. 



Group Trioxychoidea. 



The greater part of the carapace of a species of Cycloderma 

 was obtained from Bed 21 at Kachuku. The parts preserved 

 are : the nuchal bone ; the seven anterior neurals and part of the 

 eighth ; all the costals on the right side — except the outer end of 

 the second, while on the left side the second, the seventh, and 

 parts of some others are wanting. The surface of the carapace 

 seems to have been moderately convex ; its anterior (nuchal) 

 border is slightly concave, but beyond this the outline is broadly 

 convex, the shell widening as far back as the third costal. Behind 

 this it narrows to the seventh costal, where its border is even 

 slightly concave ; from this point it is evenly convex to the 

 postero-lateral convexities, between which the posterior border is 

 slightly concave. (See PI. XXVII, fig. 1.) 



The nuchal is considerably wider than long, and its anterior 

 border is slisrhtlv concave. From about the middle of its ventral 

 face, a short distance on each side of the middle line, a pair of 

 strong ridges curve outwards and apparently terminated in a 

 plate-like process overlapping the anterior part of the ventral face 

 of the first costal ; this ventral plate of the nuchal is separated 

 from the edge of the main body of the bone by a sharp notch. 

 There seems to have been no emargination of the posterior border 

 for the reception of the anterior end of the first neural. 



There were eight neurals. The anterior two are between the 

 first costals; they are of peculiar form, the first, which may perhaps 

 be regarded as a pre-neural, being asymmetrically developed as a 

 small irregularly-hexagonal plate, bounded on the left by the left 

 costal, but almost completely cut off from the right costal by an 



1 Ann. Mag. Nat. Hist. ser. 7, vol. xi (1903) p. 115, and A. von Eeinach 

 in Abhandl. Senckenb. Naturf. Gesellsch. vol. xxix (1903) p. 1 ; see also 

 E. Dacque in Geol. Palaont. Abhandl. Jena, vol. xiv (1912) p. 308 [36]. 



2 Geol. Mag. dec. 4, vol. vii (1900) p. 1. 



Q. J. G. S. No. 278. o 



