134 MOLLUSCA FROM THE GREAT OOLITE. 



and inferior borders rounded, posterior border lengthened and nearly straight ; area wide, 

 flattened, finely striated transversely, and divided into three portions by as many faintly 

 traced carina?, or rather as many lines of minute closely-arranged equal and regular tuber- 

 cles, those of the inner carina, being elongated into as many varices or plications ; there is, 

 likewise, a median divisional groove, which is immediately adjacent to and parallel with the 

 tubercles of the median carina. The clavellated portion of the shell has a numerous series 

 of rows of concentric closely-arranged but not very prominent tubercles, the larger tuber- 

 cles being towards the middle of the curvature ; they are distinct, usually rounded, closely- 

 arranged (15 or more being contained in a row), the number of rows in adult shells being 

 about 20, the whole of which are distinctly tuberculated ; the lines of growth upon the 

 sides of the shell are fine and distinct. The dimensions are equal to the largest examples 

 of the clavellated Trigoniae. The species which approximate most nearly to our shell are 

 T. perlata, Ag. T. Bronnii, Ag. T. muricata, Goldf. and T. clavellata, Sow., it having 

 usually been mistaken for the latter shell. 



T. perlata has the umbones more recurved ; the tubercles upon the carinas are much 

 larger, and those of the median carina have in addition a series of transverse varices which 

 are absent in T. decorata. T. Bronnii has the apex more elevated, it is destitute of the 

 inner varices upon the area ; the sides of the shell have a less numerous series of rows of 

 tubercles, the tubercles being larger. 



T. muricata has the area much smaller and more narrow ; the lanceolate post ligamen- 

 tal space is smooth ; the costse upon the sides of the shell are distinctly elevated, the tu- 

 bercles being more prominent and more distantly arranged in the rows. 



T. clavellata has the figure more elongated and rostrated posteriorly ; the umbones are 

 much more recurved ; the superior border of the area is distinctly concave ; the lanceolate 

 space is of great size, and the inner carina is destitute of varices ; the sides of the valves 

 have the rows of tubercles fewer, the tubercles more elevated and more distantly arranged 

 in the rows ; the general convexity of the valves being greater than in T. decorata. 



The specimen forwarded to us from Yorkshire, is rather more elongated, and the costaa 

 are somewhat more prominent than obtains in specimens from Gloucestershire ; but there 

 appears to be no essential difference between them. 



Geological position and localities. — The Great Oolite of Scarborough ; it is abundant 

 likewise in the bed called Trigonia Grit of the Inferior Oolite in the Cotteswolds. 



Astarte minima, Phil. Tab. XIV, fig. 15. 



Astarte minima, Phil. Geol. York., 1, t. 9, f. 23. 



— — Williamson. Geol. Trans., 2d ser., vol. v, pt. 1, p. 240. 



Testa parvd ovato-acutd convexd; umbonibus prominulis obliquis ; superficie apice 

 lavigato; dorso striis concentricis magnis irregularibus. 



Shell small, ovately acute, convex ; umbones prominent, pointed, oblique ; margins of 



