Vol. 62.] LIMESTONE OF THE MEN DIP AEEA. 373 



Productus <p in the following characters: — (1) Broader beak; 

 (2) produced wings ; and (3) irregular and flexuous ribbing. 



Prom Pr. hemisphericus, Pr. <f> differs in its larger size, more 

 transverse form, and in the feebler definition of the wings. 



Evolution. — Pr. 0, which is highly characteristic of the Upper 

 Dibunop7iyllnm-Zone in the Mendip area, was most probably 

 formed by direct evolution from Pr. hemispJiericus. 



Peoducttjs sp., convergent with Pr. margaritaceus, Phil. (PI. XXXII, 

 fig. 4.) 



Convex valve. — Shell small, strongly convex, transverse. 

 Eeak small, pointed, and arched. Wings well-marked, but not 

 sharply separated from the rest of the valve. Ribs well-spaced, 

 broad and rounded ; increasing almost entirety by the intercalation 

 of fresh ribs, but also by very occasional forking ; the fresh ribs 

 rapidly attain the thickness of the earlier ones. Spines few and 

 scattered on the main part of the valve, but numerous on the ears 

 and hinge-line. 



Discussion. — This well-marked form, which occurs in the 

 Upper DibunopJiyllum-Zone of the Mendip area, has a strong 

 general resemblance to Pr. margaritaceus, Phil., as represented 

 in Davidson's ' Monogr. Brit. Foss. Brachiop.' (Palaeont. Soc.) vol. ii 

 (1858-63) pi. xliv, figs. 5-6 a. The general form of the shell, the 

 pointed, arched beak, and the disposition of the spines on the wings, 

 are points of marked similarity. In the character of its ribbing, 

 however, our form differs considerably from Pr. margaritaceus, 

 which has flattened ribs, increasing mainly by forking. 



Pkodtjctus aff. elegans, M'Coy. 



Specimens occurring in the DibunophyUum-Zoiie of the Mendip 

 area are practically identical with Pr. elegans, as described and 

 figured by M'Coy, in ' Brit. Pal. Foss.' 1855, p. 460 & pi. iii h, fig. 4. 



Chonetes. 



Chonetes aff. comoides (J. Sow.). 



Compare Vaughan, Quart. Jouru. Greol. Soc. vol. lxi (1905) p. 295 & 

 pi. xxvi, fig. 4. 



The form abounding in the Lower Dibunophyllum-Zone of the 

 Mendips is identical with that which characterizes the same horizon 

 in the Bristol area. 



The form which occurs abundantly at the top of S x and the base 

 of S 2 in the Mendip area is practically identical with the D, form 

 in external characters ; but the shell is thinner, and the muscular 

 impressions much shallower, in the Seminula-Zone form. 



The specimens of Chonetes aff. comoides which occur commonly 



