﻿288 DR. A. VATJGHAN ON ME PALJ30NTOLOGICAL [May I905, 



The only well-known species of Carboniferous brachiopoda to which 

 Productus bassus bears any appreciable resemblance is Pr. aculeatus 

 (cf. Davidson, ' Monogr. Brit. Foss. Brach.' vol. ii, 1858-63, Pal. Soc. 

 p. 166 & pi. xxxiii, tigs. 18-19). 



Productus cf. Martini (Sow.). (PI. XXV, figs. 2 & 2 a.) 



Convex valve. — The general form is approximately that of 

 the specimen figured by Davidson op. cit. pi. xliii, figs. 2 a & 2b, 

 under the name of Productus semireticulatus, but the valve is more 

 elongate than in those figures, although never geniculate. The 

 sides and medial area are remarkably flattened, so that a section 

 through the hinge-line intersects the valve in a rectangle. There 

 is seldom a distinct sinus. The ribbing varies from striate (when 

 the surface appears nearly smooth) to close and thread-like (as in 

 our figure). The spines are few and scattered. Semireticulation 

 is usually well-marked. 



Concave valve. — This valve is truly geniculate, the beak-region 

 being almost perfectly square and flat. Concentric wrinkles extend 

 over the whole of the flat surface of the valve. 



Size. — The concave valve is usually less than 25 millimetres 

 square. 



Discussion. — The type-form of Productus Martini (Sow.), as 

 figured in ' Mineral Conchology ' vol. iv (1823) pi. cccxvii, figs. 2-4, 

 and as represented in Davidson op. cit. pi. xliii, figs. 7 & 7 a, 

 resembles Productus cf. Martini in its : — (1) square cross-section ; 

 (2) elongate form ; (3) semireticulate ornament ; (4) small number 

 of spines. But Sowerby's type differs very considerably from our 

 form in (1) the marked geniculation of the convex valve ; and 

 (2) the skirt-like extension of the convex valve. 



Since I have not met with Sowerby's form in the Bristol area, I 

 do not know whether Productus cf. Martini is genetically related 

 to the typical Pr. Martini (Sow.). If both forms occur on a con- 

 tinuous evolutionary line, the abnormal development of shell in 

 Sowerby's type would suggest that the true Pr. Martini represents 

 a late stage, whereas Pr. cf. Martini is certainly an early stage. 



Differences. — The square-cut sides and rectangular section, as 

 well as the much smaller form, distinguish this species from a 

 tj-pical Productus semireticulatus. 



Variety. — Certain specimens from Z t have coarser ribbing and 

 a distinct sinus, with a few symmetrically-placed large spines. 

 This variety resembles Pr. longispinus, but it differs from that 

 species in its more considerable size, and in the fact that the median 

 section is always more rounded in the neighbourhood of the beak. 



Mutation. — The evolution of Productus semireticulatus from this 

 form takes place during Z 2 , and is almost insensible. There can, 

 therefore, be no hesitation in assigning Pr. cf. Martini to the gens of 

 Pr. semireticulatus ; but, since the typical Pr. semireticulatus is only 

 found above Z 2 , whereas the semireticulate Producti, found below 

 that zone, belong almost exclusively to the Pr. cf. Martini section, 

 it is useful to maintain the separate sections. 



