160 CARBONIFEROUS LAMELLIBRANCHIATA. 



The valves are regularly curved from above downwards, and before backwards, 

 the latter part of the curve being less rapid than the former on account of the 

 greater length. The shell is rapidly compressed into the posterior-superior angle 

 above the line running from the posterior edge of the umbo to the posterior- 

 inferior angle ; the dorsal slope becomes hollow on section. There is no oblique 

 ridge, but the rest of the valve passes with a rounded gradual curve into the 

 compressed portion. 



Interior. — The anterior adductor scar is almost obsolete, and is remote from 

 the margin in the hollow of the anterior portion of the umbonal curve. The 

 posterior scar is situated some distance from the posterior-superior angle in the 

 hollow of the posterior slope. The hinge-plate is expanded in front and behind. 

 In front there are four oblique short teeth (de Koninck) ; behind, two or three 

 slightly diverging elongate teeth, subparallel to the hinge-line, but terminating 

 some distance from the posterior-superior angle, at which point the hinge-plate is 

 quite smooth, and the opposite surfaces of the two valves are in contact for some 

 little distance. The pallial line is entire. 



Exterior. — The surface of the valve bears very fine, close-set concentric lines, 

 hardly visible to the naked eye, decussated by fine, regular radiating striae, most 

 marked at the posterior end, and especially at the posterior-superior angle. 



Dimensions : 



Antero-posteriorly. Dorso-ventrally. 



PI. XI, fig. 22, M'Coy's type 30 mm. 21 mm. , 



Localities. — England : the Carboniferous Limestone of Castleton, Derbyshire, 

 and Narrowdale, Staffordshire. Ireland : the Limestone of Little Island, near 

 Cork. 



Observations. — The original of M'Coy's Psammobia decussata is in the Griffith 

 Collection of the Dublin Museum of Science and Art. It is a left valve, PI. XI, 

 fig. 22, and I am permitted to figure it by the kindness of the authorities. De 

 Koninck pointed out, in the Supplement to his earlier work on the Carboniferous 

 Fossils of Belgium (op. supra cit.), that the shell has no affinity at all to Psammobia, 

 and really belonged to the Area family. Owing to the name " Area decussata" 

 being already in use by Minister, he changed the specific name to A. anatina. In 

 his latest work, however, recognising that the Area-like shells of the Carboniferous 

 period had a distinct and characteristic hinge-apparatus, he resumed M'Coy's 

 specific name as Parallelodon decussatus. The type shell is much larger than that 

 figured by de Koninck, and is larger than any that I have been able to study. It 

 is not very well preserved. From M'Coy's figure the original would appear to 

 have been drawn from a right valve, but in all his figures Prof. M'Coy's artist did 

 not allow for the reversal of the figure during the process of lithographing. 



I have not yet been able to obtain specimens which show any details of the 



