BRITISH SILURIAN BRACHIOPODA. 163 



58. Pentamerus rotundus, J.de C. Sow. Dav., Sil. Mon., PL XV, figs. 9 — 12 ; Sup., 



PI. IX, figs. 6 to 9. 



Since this species was described at p. 150 of my ' Siliirian Monograph,' a number 

 of specimens have been obtained by Mrs. R. Gray from the Upper Llandovery at 

 Penkill, in Ayrshire, They occur seemingly more often in separate valves, but sharply- 

 defined internal casts, showing the inner surface of both valves, have sometimes been 

 met with in the same locality ; these show the septa and chambers, as well as the 

 muscular impressions, in a very beautiful and clear manner, as may be seen by a glance 

 at the figures in PL IX of this Supplement. 



The dorsal was more convex than the ventral valve, at about half its length the 

 mesial fold commences to rise, and is generally composed of four strong ribs with wide 

 slopes. On the lateral lobes of this valve are two or three short, slightly projecting ribs 

 observable near the margin. In the ventral valve the sinus commences at about half 

 the length of the valve and is of moderate depth. Three ribs usually occupy the sinus. 



The largest specimens I have seen from Ayrshire did not exceed 1 inch in length and 

 breadth by 8 or 9 lines in depth. P. rotundus has also been found by Mr. R. Philip 

 in the Upper Llandovery at Minsterley, in Shropshire ; where it is associated with 

 Fentamerus oUonous and P. undatus. 



59. Pentamerus globosus, /. de C. Sow. Dav., Sil. Mon., PL XIX, figs. 10 — 12; 



Sup., PL IX, figs. 21 to 24. 



This species is described at p. 156 of my ' Silurian Monograph.' Since then a large 

 number of specimens of the fossil have been collected by Mrs. R. Gray from the Upper 

 Llandovery at Penkill, in Ayrshire. The shell is often nearly circular, globose, and as 

 wide as long. Many examples show neither fold nor sinus, while others possess a 

 rounded fold of small elevation in the dorsal valve, beginning to rise at about half the 

 length of the valve. 



P. (globosus does not appear to have much exceeded 7 lines in length by about 8 in 

 breadth and 5 or 6 in depth, it is more nearly allied to Pent, linguifer than to any 

 other species of the genu soccurring in our Silurian rocks. The shell has also been 

 found by Prof. T. M'Kenny Hughes in the Upper Llandeilo at Penlan, Llandovery. 



