UNDULATE. 77 



and concave upon their upper sides ; their direction has a slight curvature obliquely 

 downwards towards the carina ; the first-formed four costse are simply horizontal, the 

 succeeding costag are bent suddenly upwards at their posteal extremities, forming a series 

 of right angles, one of which proceeds from every second costa. The entire number of 

 costse in adult shells is nineteen or twenty in a specimen eight lines in height. In aged 

 specimens the few last-formed costse are small, or indistinct, broken, or undulating, and 

 their posteal extremities become somewhat nodulous when the whole of the ornamenta- 

 tion becomes nearly effaced. 



The height and lateral diameter are nearly equal ; the diameter through both the 

 valves is one fifth less. The size varies from three to ten lines across the valves. 



Stratigrapliical positions and Localities. The Great Oolite of Box, near Bath, has 

 produced numerous examples of varying dimensions and stages of growth. Mr. 

 Cunnington has also obtained it in the Cornbrash of Westbrook and Trowbridge. 

 Specimens are in the British Museum ; in the Museum of Practical Geology, Jermyn 

 Street ; the Woodwardian Museum, Cambridge ; in the Cabinet of Mr. Walton, of Bath ; 

 Mr. Cunnington, of Devizes ; and in my own collection. France Luc, Calvados, in the 

 Great Oolite. 



Trigonia undulata, FromJierz. PI. XVI, figs. 9, 10, 11 ; PI. XVII, figs. 5, 6. 



Trigonia undulata, Fromherz. Agassiz, Trigonies, 1840, p. 34, tab. x, figs. 14, 16, 



exclude tab. vi, fig. 1. 

 — ARATA, Lycett. Suppl. Monogr. Gr. Ool. Moll., Pal. Soc, 1863, p. 52, 



tab. xl, fig. 2 (variety). 



Shell sub-ovate or ovately trigonal, moderately convex; umbones little produced, 

 obtuse, nearly erect, antero-mesial ; anterior border moderately produced, curved ellipti- 

 cally with the lower border; hinge-border lengthened, nearly straight, sloping obliquely, 

 forming a considerable angle with the posteal extremity of the area. Area flattened but 

 slightly convex, the plane of its surface differing only slightly from that of the other 

 portion of the valve ; its breadth is equal to one third of the costated surface ; it is 

 traversed by large transverse plications which become prominent costella upon the upper 

 third of its length ; the marginal carina has little prominence ; the median furrow is well 

 defined, but there is no median carina ; the posteal extremities of the costellse form a 

 tuberculated inner carina. The escutcheon is narrow and much excavated, its superior 

 border is raised ; one of our specimens has several oblique varices across its surface, but 

 this appears to be an abnormal feature. The rows of costse are numerous, small, and 

 not quite regular in their arrangement, the first-formed six rows are plain, slightly curved 

 and acute ; the succeeding rows descend obliquely from the anterior side, and about the 



11 



