166 CRETACEOUS LAMELLIBRANCHIA. 



more nearly equal in size. Surface with concentric ridges or growth-lines, which 

 sometimes develop into small spiny projections where they cross the i-ibs. Anterior 

 ear large, rising dorsally, with a deep byssal sinus. 



Left valve rather more convex, with usually from thirty-eight to forty-fovir ribs 

 at the margin, where they are separated by very narrow grooves. Most of the ribs 

 are of nearly equal size, Imt occasionally smaller ones occur ; they are rounded on 

 the early parts of the valve, but become flattened and broader ventrally, and also 

 anteriorly and posteriorly. Secondary ribs are introduced at a short distance 

 from the umbo. The grooves become narrow ventrally. Surface of ribs and 

 grooves with close-set, concentric, linear ribs, which are better marked near the 

 umbo than elsewhere. The bent antero- and postero-dorsal margins are without 

 ribs, but have numerous radial strige ; these striee appear on some of the ribs also. 

 Anterior ear more elevated and larger than the posterior ; both witli a few broad 

 radial ribs. 



Measurements : 



(1) (2) (3) 



Length . 42 . 39 . 35 mm. 



Height . 50 . 49 . 42 „ 



(1) Ceuoiuauiau (Bed 11), Duuscombe. 



(2) Cenomanian, Wilmington. 



(3) Ceuonaaniau (A. Mantelli zone), Beer Head. 



Affinitieti. — The ribs in this species are more numerous as a rule, and the 

 concentric ornamentation much less well-developed than in P. Espaillaci, d'Orbigny. 

 The ribs are much more numerous than in P. fissicosta, Etheridge, and the radiating 

 strise much less developed, being apparently confined to the anterior and posterior 

 part of the valves. 



BemarJcs. — The English specimens are smaller than those figured by Matheron 

 and d'Orbigny, and, as pointed, out by Jukes-Browne, agree better with the figures 

 than with the descriptions given by those authors, but specimens obtained from 

 France leave no doubt as to the identity of the English form with Matheron's 

 species. I have not seen the arrangement of the ribs shown in Matheron's section 

 (fig. 3), but there is sometimes an alternation of large and small ribs. 



Types. — Cenomanian (Chert Beds) of Les Martigues, Uchaux, and Mornas Sault. 



Distribution. — Cenomanian of Wilmington. Cenomanian (Beds 10, 11) of 

 Beer Head, and (Bed 11) of Dunscombe. Chloritic Marl of Melbury (Dorset) 

 and Maiden Bradley. 



