ANIMALS. 173 



hinge-line, as those of CucuUeea (vide PI. XV, fig. 8), and finely sulcated. It was the 

 former character that led Mr. J. de C. Sowerby to place it in the genus just named. 



Byssoarca striata occurs at Tunstall Hill and Humbleton Quarry, in Shell-limestone. 

 Professor Phillips appears to have discovered it at Ferry Bridge. Dr. Geinitz records its 

 occurrence in the under Zechstein of Corbusen, near Ronneburg ; and in the Zechstein- 

 dolomite of Poessneck, Gliicksbrunn, and at Wartberg, near Seebach, in Thuringia. 



Byssoarca tumid a, /. Sowerby. Plate XV, figs. I, 2, 3, 4, 5. 



Arca ANTiauATA, Linn. J. B. Taylor, Surtees's Durham, vol. i, p. 249, 1816. 

 Casts of Aec^e, Winch. Trans. Geol. Soc. Lond., vol. iv, p. 10, 1817. 



— — ,, Conybeare and Phillips's Geology, p. 305, 1822. 



Arca tumida, J. Sowerhy. Mineral Conchology, vol. v, p. 116, pi. 474, fig. 3. 



— — „ Fleming, Brit. Animals, p. 399, 1828. 



— — „ Sedgvrick, Trans. Geol. Soc. Lond., 2d series, vol. iii, p. 119, 



1829. 



— — „ De laBeche, Geol. Man;, p. 384, 1831; Germ, Transl. p. 



459, 1832 ; 3d Eng. ed., p. 573, 1833. 



— — „ Phillips, Encyc. Met., vol. iv, p. 616, 1834. 



— — „ Thomson, Outlines of Mineralogy, &c., vol. ii, p. 314, 1836. 



— — „ Morris, Catalogue, p. 78, 1843. 



— — „ De Verneuil, Bull. Soc. Geol. de France, 2"° serie, vol. i, 



p. 32, 1844. 



— — „ Geol. Russ., vol. i, p. 224, 1845. 



— — „ Tennant, Strat. List, p. 88, 1847. 

 Byssoarca tumida, J. (Sow. King, Catalogue, p. 11, 1848. 



Arca — „ Howse, Trans. T. N. F. C, vol. i, p. 247, 1848. 



Diagnosis. — " Transversely elongated, gibbose, costated (?) ; anterior side pointed ; 

 marginal sinus short, deep ; beaks distant." (J. Sowerby.) 



Byssoarca tumida differs from the last species principally in being more tumid, 

 shorter transversely, and more rounded at the ventral margin. In general the ribs 

 (which were only slightly indicated on the cast examined by Mr. Sowerby) are simple ; 

 but occasionally specimens occur in which they are distinctly divided : they are more 

 or less granulated or nodulous : those posteriorly situated are generally so ; the anterior 

 and median, only occasionally. Sometimes specimens occur with the valves beauti- 

 fully decussated, arising from the ribs and intervening furrows being crossed with 

 rather prominent incremental lamellse. The cartilage-grooves on the areas are very 

 fine, and uniangulated under the umbones (vide PL XV, figs. 2, 3). The byssal 

 opening is rather large (fig. 5). 



This species appears to have had a wider range than the last, being found at 

 Humbleton Quarry, Hylton North Farm, Tunstall Hill, Dalton-le-Dale, and Ryhope 

 Field-House Farm, in Shell-limestone, and somewhat common. Having referred the 

 specimen figured by Geinitz in the ' Versteinerungen' with a doubt to Byssoarca striata, 



