928 DK F. R. C. REED ON THE 



but the type of this species which comes from Penwhapple Glen has only 2 or 

 3 plications forming the fold, and they are coarse and equal, and the shell is less 

 transverse in shape. Our shell much more resembles P. thraivensis, from the Starfish 

 Bed (see p. 930), and P. angulosa (Tornquist),* but it shows some variation and 

 irregularity in the plications on its surface {e.g. fig. 34). 



Parastrophia rotunda (Sowerby). 

 (Plate XXII, figs. 35-41.) 



1839. Atrypa rotunda, Sowerby, in Murchison's Silur. Syst., p. 629, pi. xiii, fig. 7. 



186S. Pentamerus rotundus (Sowerby), Davidson, Mon. Brit. Foss. Brack., vol. iii, pt. vii, p. 150, pi. xv, 



figs. 9-12. 

 1883. Pentamerus rotundus (Sowerby), Davidson, op. tit., vol. v, Silur. Suppl., p. 163, pi. ix, 



figs. 6-9. 

 H883. Pentamerus galeatus (Sowerby), Davidson, ibid., pJ164, pi. ix, figs. 25, 25a (non Pentamerus 



galeatus, Davidson, op. cit., vol. iii, p. 145, pi. xv, figs. 13-23). 



The diductor muscles are not shown in Davidson's fig. 7 of a pedicle-valve from 

 Penkill, though they are well seen in the actual specimen, of which the figure is a 

 poor restoration. These diductors are flabellate, triangular, lie close up against the 

 septum, are radially striated, and are about one-fourth (or rather less) the length of 

 the valve. The fact that the sinus is on the pedicle-valve distinguishes this shell 

 from Pentamerus galeatus, as well as the low small and less conspicuous beak. In 

 Davidson's figure (fig. 9) of the internal cast of the brachial valve from Penkill there 

 should be shown two slightly convergent nearly vertical septa, very closely placed, 

 and making a very narrow elongated spondylium extending about half the length of 

 the valve, beyond which the septa seem to unite into one and be slightly prolonged 

 as a single vertical plate. The adductors also in this valve are not as he shows, but 

 form a regularly oval area bisected by the spondylium and composed of two pairs of 

 rather small scars in contact, the anterior ones being the smaller, but not transverse, 

 as he depicts. The dental plates in the pedicle-valve are short, convergent at about 

 45°-60°, making a small short spondylium, and the median supporting septum is 

 shorter than that in the opposite valve. 



It is probable that the shell from Penkill referred by Davidson to Pentamerus 

 galeatus (op. cit., pi. ix, figs. 25, 25a) belongs to this species. 



As Hall and Clarke pointed out,t Atrypa rotunda, Sowerby, must be placed in 

 the genus Parastrophia, its internal characters, position of fold and sinus, am! 

 external features much resembling P. reversa (Billings) | of the Clinton Group. 

 P. rotunda was first described as Atrypa by Sowerby § from the Wenlock Group, 

 and Davidson (op. cit., iii, p. 151) was perplexed about its proper generic reference. 



Horizon. — Penkill Group. 



Locality. — Penkill. 



* Lindstrom, Fragm. Silur. (1880), p. 23, t. xiii, figs. 14-19. 



+ Hall and Clarke, op. cit., Brach., ii, p. 221. J Hall and Clarke, ibid., pi. lxiii, figs. 8 14. 



§ Sowerby, in Murchison's Silurian System., pi. xiii, fig. 7 ; Davidson, op. cit., vol. iii, p. 150, pi. xv, figs. 9-11. 



