70 PERMIAN FOSSILS. 



the imperforate valve ; it is also to be seen both posteriorly and anteriorly attached in 

 Magas pumilus -^ while in the singular Pachyrhynchus roseus it is simply united to the 

 anterior end of a corresponding plate ; whereas in Terebratula (?) Natalensis it seems 

 to be reduced to two erect processes springing from the centre of the imperforate 

 valve. 



Perhaps the loop, in no case, exists under so enlarged a form as in Strigocepkalus 

 Burtini, — a shell on which I feel it necessary to make a few observations corrective 

 of what I have elsewhere stated respecting its internal structure.^ Having lately 

 succeeded in exposing the interior of a specimen, which for a long time defied all my 

 efforts to disclose, I am now enabled to state, that the internal structure of this 

 species does not consist of two parts folded, so as to form a pair of " symmetrical 

 subgyrate appendages," as I formerly stated ; but it is in reality a single piece, having a 

 tolerably close agreement with the loop of certain Terebratulida (PI. XIX, fig. 1 d). 

 The crura have certainly an unusual peculiarity ; their modification from the ordinary 

 form, however, is not difficult of explanation. Shorten the ascending stems (c), and 

 conjoin them with the descending portions of the loop {d), — the resulting form will be 

 precisely the same kind of projecting crura seen in PI. VI, fig. 45 e, and PI. XX, 

 fig. IIE. 



The figures which M'Coy and Davidson have given of the spirals or labial supports 

 of BeWtyris and Triyonotreta, show very clearly that they are connected with the 

 crura, and these again with the crural base near the socket-ridges, as in Terebratulida ; 

 and it affords me much pleasure in finding my own observations on the spirals of 

 Belthyris cristata fully corroborating their representations. 



The imperforate or small valve of many genera has a plate holding a position 

 similar to the one already noticed, running along the medio-longitudinal line of the 

 opposite valve (PI. XIX, figs. 3 b, &c.). To distinguish this plate from the latter, it is 

 proposed to term it the dorsal median plate. In Waldheimia Australis (PI. XX, fig. 11 G^) 

 and several other species, it supports the crural base ; in Camerophoria multiplicata 

 (PL VIII, figs. 3, 4/) it steadies a spatula-shaped process, which will hereafter be 

 described ; and in Mayas pumilus it affords support, at its anterior end, to a modified 

 form of a probably homologous structure ; but in Prodtictus (PI. XI, fig. 10 b), which has 

 neither a crural base nor loop, its use appears only to keep certain muscles separated 

 from each other. The dorsal median plate apparently subserves the latter purpose in 

 Leptcena (PI. XX, fig. 7 b), and some other shells ; but in another form, Strophomena 

 Butertrii, there seems to have been a failure in this respect, since the muscles were 

 implanted on lamellar fulcra, which curve over it, and coalesce, forming, as it were, 

 a complete arch. 



' Vide Note sur le Magas pumilus, par MM. Th. Davidson at Boucliard-Chantereaux, Bulletin de la 

 Soc. Geol. de France, 2""' s^rie, torn, v, pp. 139-41, pi. ii. 



^ Annals and Magazine of Natural History, vol. xviii, p. 88. 



