﻿290 DE. A. VAUGHAN ON THE PALJIONTOLOGICAL [May I905. 



at first thin, but they soon acquire the same thickness as the ribs 

 of the earlier series. The new ribs remain thin, longest, on the 

 flanks. Concentric wrinkles are usually conspicuous on the wings, 

 and are often indistinctly continued on to the beak-region. The ribs 

 are finely crenulated by regularly- spaced annulations. The muscular 

 impressions are of the general type seen in strongly-convex Product* 

 (see text-fig. 3 & note below, p. 293). 



Resemblances and differences. — This form, which is very 

 common in S v has always been recorded as Productus giganteus, 

 on account of its large size, extreme convexity, and fine ribbing 

 (characters which have also been responsible for the registration of 

 Clionetes cf. comoides under the same name). 



Productus 6 differs from Pr. giganteus in the flatness of the wings, 

 the shorter hinge-line, the narrower beak, and a distinct trace of 

 semireticulation, and in these characters it shows an approximation 

 to Pr. semireticulatus. In association with Productus 6 are found 

 large examples of Pr. semireticulatus, which only differ from the 

 former in the flattening of the medial area of the convex valve and, 

 consequently (see below, p. 293), in the more anterior position of 

 the adductor-impressions, as well as in the stronger semireticulation. 



In the general type of ribbing, the uniform narrowing of the 

 valve towards the beak, and the steep flanks, Productus 6 resembles 

 Productus Cora [Day.], and especially the mutation C. Productus 6 

 differs, however, in the broad flattened wings and in the thick 

 test. 



Evolution. — It seems probable that Productus 6 represents a 

 complex type, formed by evolution from Pr. Cora [Dav.] mut. C, 

 along a line characterized by strong assimilation to Productus 

 semlreticu latus. 



Discussion. — The great size of the shell and the thick test 

 suggest that Productus is the culminating form of a line of 

 evolution, and this suggestion is somewhat confirmed by the fact 

 that, between the top of S x and the top of S.,, where the true 

 Productus giganteus makes its first appearance, large Producti are 

 absent in the Bristol area. 



I am, however, doubtful whether Productus 6 is more than a 

 rather striking divergent from the gens of Pr. Cora [Dav.], for it 

 seems almost impossible to separate the young form of Productus 6 

 from the mutation of Pr. Cora [Dav.], which is figured in Proc. 

 Nat. Soc. Bristol, n. s. vol. x (1903) pi. ii, fig. 4, and occurs at nearly 

 the same horizon. For this reason, it seems better to await more 

 material before creating a new specific name. On the other hand, 

 Productus 6 has an important stratigraphical value, wherefore it is 

 essential, for my present purpose, that it should be described and 

 figured. 



Peodtjcttjs Coea, d'Orb., Davidson. (PI. XXV, figs. 4-4 b.) 



This is a laige gens which includes, as a typical member, the 

 form figured in Davidson's ' Monogr. Brit. Foss. Brach.' vol. ii 

 (1858-63) Pal. Soc. pi. xxxvi, figs. 4-4 6. All the following 



