﻿296 DK. A. VAUGHAN ON THE PAL^EONTOLOGICAL [Maj T I905, 



examples of Productus giganteus at the same level, which have 

 developed a narrow hinge-area and show distinct traces of tubuli, 

 beneath the area, in communication with a series of spines along the 

 cardinal ridge (Ohonetiproductus). 



Leptena. 



Leptena analoga (Phil.). 



As interpreted in Davidson's ' Monogr. Brit. Foss. Brachiop.' vol. ii (1858- 

 63) pi. xxviii, figs. 1-3 (Strojihomena analoga). 



Orthotetes. 



Oethotetes oeenistria (Phil.). 



As interpreted in Davidson's ' Monogr. Brit. Foss. Brachiop.' vol. ii (1858-63) 

 pis. xxvi & xxvii (Streptorhynchus crenistria). 



The forms of this large gens undergo continuous modification, and 

 consequently specimens found at two horizons, separated by a con- 

 siderable vertical interval, are conspicuously distinct. To this gens, 

 at least throughout the areas which I have myself examined, the 

 labour-saving formula — ' there are no greater differences between 

 the different varieties than could be found between individuals 

 occurring in the same bed '■ — most assuredly does not apply. The 

 individuals seen in a particular bed are usually exact duplicates of 

 one another, and, all alike, differ from the forms occurring in an 

 earlier or later zone. 



Without increasing very considerably the number of plates, it is 

 impossible to give a satisfactory account of the mutations of the 

 gens ; I shall consequently content myself with the description of 

 two striking mutations only. 



Mutation Z characterizes the Zaphrentis-zone, and especially Z 2 . 



The brachial valve is globose; the hinge-area is broad and 

 StreptorJignchiis-like ; the hinge-line is usually produced into short 

 ears ; the ribs are close- set, and the alternation of ribs is made 

 out with difficulty. 



The convexity of the brachial valve and the type of ribbing make 

 it often difficult to distinguish this form from Schizophoria resupinata, 

 with which it is found associated. This is another instance of the 

 approximation of co-existing cognate forms. 



Mutation C. Characteristic of the laminosa-subzone. 



This mutation is typically Strojohomena-like. The brachial valve 

 is convex, the hinge-area narrow, and the alternation of ribs very 

 distinct. There is often a medial depression in the brachial valve 

 (cf. Orthotetes crenistria, var. Kellii, M'Coy). 



Schizophoria. 



Schizophasia restjpinata (Martin), Davidson. 



Davidson, in his ' Monogr. Brit. Foss. Brachiop.' vol. ii (1858-63) 

 pi. xxix, figs. 1 & 2 (Orthis resupinata), represents the form l : but 



1 The Bristol forms, common in Z 2 , have, however, a much deeper brachial 

 valve. 



