158 CARBONIFEROUS LAMELLIBRANCHIATA. 



lines of growth, parallel to the margins. Under the microscope the valves are 

 seen to be covered by minute radiating lines, so that the shell has a very finely 

 reticulated appearance. On the posterior slope the radiating lines are strongly 

 marked, six to eight in number ; but they become gradually obsolete as they pass 

 over the oblique ridge. 



Dimensions. — The type of M' Coy's shell measures — 



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

 PI. XI, fig. 5 . 22 mm. 9 mm. — 



PI. XI, fig. 8, from Dairy 21 mm. 8*5 mm. 5 mm. 



Localities. — England : Mallerstang ; shale above the Main Limestone, Millgill, 

 Askrigg. Scotland : Linn Spout, Dairy, in a bed of shale below the Linn Lime- 

 stone, and Orchard Quarry, Upper Limestone series ; Craigenglen, Campsie, Lower 

 Limestone series ; Wilkieson, Fife. Ireland : Middle Limestone or Calp series, 

 Manor hamilton. Coal-measures eight miles south-west of Kilkee, Co. Clare. 



Observations. — There is, fortunately, no doubt about this species, as the type 

 which I am permitted to refigure (PI. XI, fig. 5) is preserved in the Griffith Col- 

 lection of the Dublin Science and Art Museum. The species is fairly common at 

 Linn Spout, Dairy (where I was taken by Mr. Smith of Kilwinning). A single 

 specimen marked "Mallerstang" is in the collection of Miss Jane Donald, of 

 Carlisle. P. semicostatus is recorded in Young and Armstrong's ' Catalogue of 

 the Fossils of the West of Scotland ' under " A. reticulata." 1 Only five species 

 of this genus are listed in the above work — A. arguta, Phillips ; A. faba, de Kon. ; 

 A. fimbriata, de Kon. ; A. Lacordaireana, de Kon. ; and A. reticulata, M'Coy. 



As Prof, de Koninck observes that his P. meridionalis somewhat resembles 

 P. semicostatus ; but in that species the form and general shape differ, there are no 

 ribs on the posterior slope, and it is altogether more tumid. 



P. semicostatus does not seem to attain any great size, and I have never as yet 

 met with it in limestone at Dairy ; it is associated with P. Geinitzi, Nucula 

 gibbosa, Fleming, Nuculana attenuata, Fleming, sp., N. Igevirostrum, Portlock, sp., 

 Edmondia uuioniformis, Phillips, sp., and a fairly rich and varied Lower Carboni- 

 ferous Molluscan Fauna, although the Linn Limestone is at the top of the 

 " Carboniferous Limestone series " of Scotland. Many of the species of this fauna 

 occur in the lower portion of the Calciferous Sandstone series of Fifeshire, but 

 P. semicostatus has not yet been found at such a low horizon. Although P. semi- 

 costatus has much the same shape and size as P. Geinitzi, the other species with 

 which it occurs, it is easily distinguished by the presence of strongly marked 



1 Dr. John Young writes me that he knows of only two species from the Carboniferous rocks of 

 the West of Scotland, and that Armstrong was responsible for the list. Dr. Young kindly submitted 

 his specimens to me, and I identify the A. reticulata as P. semicostatus ; and I think it probable that 

 A. faba of the catalogue is the P. Geinitzi, de Koninck. 



