170 CARBONIFEROUS LAMELLIBRANCH1ATA. 



above downwards, and nearly square with the upper and lower borders. The 

 hinge-line is straight, as long as the greatest transverse diameter of the shell. 

 The umbones are small, narrow, elongate, incurved, hardly raised above the hinge- 

 line, close but not contiguous, and situated in the anterior fifth of the hinge-line. 

 Internal to the umbo is a narrow, elongate, ligamental area, broader in front, and 

 becoming obsolete posteriorly. The valve is evenly swollen, but towards the 

 posterior-superior angle becomes compressed and even hollowed. 



Interior. — The anterior adductor scar is normal in position ; the posterior 

 one is circular and placed low down and well within the margin. The hinge 

 consists in front of five very small teeth inclined obliquely downwards and 

 backwards, situated on a triangularly expanded plate ; but I have not been able to 

 ascertain the existence of posterior lateral teeth, which, if present, must be very 

 short and terminate only just posterior to the umbones. 



Exterior. — The surface is smooth, but under the microscope irregularly 

 concentric obsolete lines of growth are seen, which become obscurely sub- 

 imbricating on the posterior slope ; and the imbricating edge, under a very high 

 power, is dotted with fine, non-continuous, radiating striae. 



Dimensions. — PI. XI, fig. 21, measures — 



Antero-posteriorly . . . .18 mm. 



Dorso-ventrally . . . .7 mm. 



Elevation of valve . . . .2 mm. 



Locality. — Shale underneath the Linn Limestone in the Upper Limestone series 

 at Linn Spout, Dairy, Ayrshire ; Craigenglen, Campsie, Lower Limestone series. 



Observations. — This species is found in shale beneath the Linn Limestone at 

 Linn Spout, Dairy, together with P. semicostata, Nucula gibbosa, Nuculana attenuata, 

 and several other Lamellibranchs, &c. Until now it has not been recognised as 

 occurring in Great Britain. It is distinguished from other species by its 

 transverse form and apparently smooth shell, which is seen to be concentrically 

 and radiately striate only under the microscope. 



At this locality the shell is comparatively plentiful, but I am at present 

 unaware of its occurrence in other beds. 



The interumbonal ligamental area is comparatively of large size. This is also 

 the case in P. pinguis, de Koninck. 



The fact that the associated fauna with which this species occurs at Dairy 

 does not occur with it in Belgium, makes me somewhat doubtful as to the 

 correctness of my reference, although from de Koninck's description and figures 

 there is good reason to adopt the name P. Geinitzi for the Dairy shell. 



