294 BRITISH SILURIAN BRACHIOPODA. 



front, less convex visceral disk, and gradually developed, more or less regular, concentric 

 wrinkles, agreeing with Hall's figs. 3e and 3/ and fig. 4 of the same plate, referred by 

 him to L. tenuistriata of Sowerby, from which all the varieties may be easily dis- 

 tinguished by the narrow rounded front, giving the semi-oval, instead of the rhombic, 

 outline to the disk, and the inequality of the striae, as well as the narrower, oblong, 

 muscular impressions, and the distinct tubular perforation of the beak, with its corre- 

 sponding mamilla on the apex of the cast. This latter variety seems to include 

 the still more strongly pronounced form which I have called L. seniiovalis, and found 

 very constant in its characters in the Bala Limestone of the Chair of Kildare, in Ireland." 

 He then suggests that the different varieties of this species might be separated into 

 the different subgenera Leptana and Leptagonia, showing these latter to be but sections 

 of one genus. 



Strophomena deltoidea has been^well described by Prof. Hall, M. de Verneuil, Prof. 

 M'Coy, and others ; and some correct illustrations of the exterior and interior of our 

 British specimens will be found in our accompanying plate.^ 



I have also ascertained, from a minute examination of the type of M'Coy's Orthis 

 suhlavis, kindly lent to me by Sir Richard Griffith, that it is nothing more than a small 

 young example of the shell under description. Leptana plkotis, M'Coy, is also another 

 synonym of Strophomena deltoidea ; but I am not so sure with reference to his Orthis 

 undata. 



Position and Locality. In ' Siluria ' Strophomena deltoidea is limited to the Caradoc 

 or Bala period. Messrs, Ramsay, Salter, and M'Coy found the shell at Cerrig-y-Druidion 

 and Cyrn-y-brain, in Denbighshire ; also at Bala, in Merionethshire ; at Alt-yr- Anker, 

 Meifod, in Montgomeryshire ; and in sandstone at Horderly. It has been also obtained 

 by Prof. Harkness in Caradoc Limestone at Keisley, in Westmoreland. 



In Ireland it is found at Grangegeeth, County Meath, and at the Chair of Kildare, &c. 



Abroad \kit shell is common in the Trenton Limestone at Trenton Palls and Sugar 

 River, in Lewis County, and in the neighbourhood of Little Falls, &c., in the State 

 of New York. It occurs at Paggart, in Esthonia, and at Reval; in Norway, and 

 elsewhere. 



' Prof. J. Hall observes, at p. 107 of the first vol. of his 'Pal. New York,' "It is certainly often very 

 difficult to draw the line of distinction between this species and the Leptcena alternata, and more parti- 

 cularly so between this and L. camerata. Again, on the other hand, it approaches very close, in some of its 

 forms, to the succeeding species Lept. tenuistriata ; but these two, when well preserved, are clearly and 

 decidedly distinct." 



