154 CRETACEOUS LAMELLIBRANCHTA. 



i^ndial strine usually less distiuct and sometimes abseut except near the uml)0. Ears 

 slightly unequal, with ridges and radial grooves. 

 Measurements : 



A B 



(1) (2) (3) (4) (5) (G) (1) (2) (3) (4) (5) (6) (7) 



Length . r,7 118 118 120 182 191 ... 52 150 191 195 204 212 231 mm. 

 Height . 02 125 129 127 115 207 ... 52 145 179 184 192 201 220 „ 



A. Tlio liigliei- and shorter forms. (1) Tcalby Limestone : (2--G) Claxby Ironstone. 



B. The lower and longer forms. (1) Tealby Limestone : (2 — 7) Claxby Ironstone. 



Affinities. — Goldfuss' P. circularis ^ was regarded l3y Romer and Bronn as 

 identical with P. crassitesta, Romer; subsequently both were considered l)y Romer 

 as synonyms of P. eindus, Sowerby. The example figured by Goldfuss is stated 

 by him to have come from the Greensand of Dorsetshire ; but no species of this type 

 appears to have l)een found in the Greensand of that district, whereas the similar 

 form 1^. lamellosas, Sowerby, from the Portlandian, is common there, and Goldfuss' 

 figure agrees more closely with that than with P. cinrtus : this view was evidently 

 held by Morris, since lie gives 1'. circularis as a synonym of P. lamcUosns. 

 P. lamiellosus, Sowerby,^ is undoubtedly closely allied to P. cinctus ; the ornamenta- 

 tion is practically identical in both, for although the radial striae on the former are 

 usually less distinct in specimens from the Portland Limestone, they arc quite as 

 well-marked in specimens from the Portland Clay as in P. cinctus. P. ImneUosns 

 may, however, be distinguished by its greater obliquity, smaller apical angle, and 

 proportionately higher ears (especially the anterior). The shell never attains 

 the same size as P. cinctus, its average l)oing nmch less ; the height ajipears to 

 be nearly always greater than the length. 



Uemarl-s. — There are two forms of this species which differ in the proportions 

 of length and height, as will be seen from the figures, and from the measurements 

 (a and T.) given above. In one (a) the height is greater than the length, and the 

 valves are oval in outline, with a smaller apical angle and more elevated cars 

 (Text-fig. 2). The right valve is rather more convex than in the other form. 

 This form (a) was figured by Sowerby as P. cinctus. In the other form (li) the 

 length is greater than the height, and the valves are more nearly circular in 

 outline, with a larger apical angle and lower ears. The convexity of the valves 

 is less, and the anterior and posterior parts are more compressed (PI. XXVIII). 

 This form was figured by d'Orbigny as P. crassitesta. 



Since the forms a and jj agree in all the details of ornamentation, and also 



1 'Petref. Germ.,' vol. ii (183G), p. 76, pi. xcix, fig. 10. 



2 ' Min. Conch.,' vol. iii (1819), p. 67, pi. cc.vxxix. See also de Loriol and Pellat, " Mon. de IVtage 

 Portlandien de Boidognc-siir-Mer " ('Mem. Soc. Phys. et d'Hist. nat. de Geneve,' vol. xix, 1866), 

 p. 103, pi. X, fig. 4, 



