168 CARBONIFEROUS LAMELLIBRANCHIATA. 



oblique teeth ; posteriorly it is thickened, and there are two or three long sub- 

 parallel lateral teeth. The pallial sinus is entire, and not very far from the border 

 of the valve. 



Exterior. — The surface is ornamented with a large number of concentric lines, 

 hardly perceptible to the naked eye above, but somewhat more marked near the 

 lower border. These are slightly interrupted in the neighbourhood of the byssal 

 sinus ; even under the microscope the fine radiating lines, so characteristic of the 

 genus, appear to be obsolete. Shell moderately thick. 



Dimensions : 



Antero-posteriorly. Dorso-ventrally. Elevation of valve. 

 PL XII, fig. 5 (incomplete), 60 mm. 26 mm. (near umbo) 14 mm. 



PI. VIII, fig. 9 (a cast), 57 mm. 25 mm. ,, 8 mm. 



Localities. — The Carboniferous Limestone of Thorpe Cloud, Park Hill, near 

 Longnor, and Castleton, Derbyshire ; Hill Bolton, Bolland, and Settle, Yorkshire. 

 Ireland : Congleton Edge ; Dunsink, Co. Dublin ; Graigue, Co. Limerick. 



Observations. — Unfortunately the type specimen has been lost, but the species 

 is a well-recognised one, and four good specimens are preserved in the Gilbertson 

 Collection of the British Museum, three of which are figured, PI. XIV, figs. 1 — 3. 

 It has also been re-described on two occasions by de Koninck (op. sup. clt.). He 

 himself admits that one. of the specimens figured on the first occasion does not 

 belong to the species ; and even on the last occasion he says (p. 147), " Je ne suis 

 pas certain que l'espece ci-dessus represente exactement celle qui a ete figuree par 

 J. Phillips sous le nom de Cucullsea obtusa, d'apres un echantillon plus ou moins 

 defectueux [the specimen, however, judging from the figure, is quite perfect, 

 though it may be only the cast], quoique generalement, dans les collections 

 anglaises, les echantillons etiquetes sous ce nom soient identiques avec ceux 

 que je viens de decrire." The shells figured by de Koninck are not very 

 much like Phillips's original figure. They do not possess the straight inferior 

 border or truncate posterior end so characteristic of P. obtusus. Two shells 

 figured by de Koninck, pi. xxiv, figs. 19 and 34, are almost quadrate, and, 

 I think, can by no means belong to the species in question. Indeed, the figure 

 of P. tenuis, pi. xxiv, fig. 23, is much more like the P. obtusus of Phillips than 

 any of the shells figured by de Koninck under that name. 



I reproduce the original figure of Cucullsea obtusa, PI. XII, fig. 3, from the 

 drawing, and figures of three others in the Gilbertson Collection, PI. XIV, figs. 1 — 3. 

 I have a series of fragmentary specimens from the beds at the top of Thorpe Cloud, 

 Dovedale, PI. XII, figs. 4, 6, and 9, which have afforded material for the obser- 

 vation of several details in the anatomy of the shell. Most of these have portions 

 of the test preserved ; one, PI. XII, fig. 9, shows the posterior lateral teeth of the 

 hinge-plate in a cast. PL VIII, fig. 9, is the cast of a large example in the Museum 



