56 MOLLUSCA FROM THE GREAT OOLITE. 



Trigonia goldfussii, Agass. Tab. V, fig. 18, 18«. 



Lykodon lttteeatum, Goldfuss. Petref., t. 136, f. 5, 18-10. 

 Tkigonia Goldfussi, Agaz. Mem. sur les Trigonees, p. 35. 

 1 — cospidata, Soio. Min. Con., t. 507, f. 4, 5, (junior.) 



Testa plano-convexd, ovato-trigond, antice et in/erne rotundatd, postice truncatd; 

 tuberculis per series undulatas dispositis, ad carinam mqrginalem crassissimis ; carina 

 marginali tuberculis ornatd; area cardinali transversim striata; tuberculis nonnullis in 

 carina interna. (Agassiz, pro parte.) 



Shell with a moderate convexity, ovately trigonal, the anterior and inferior borders 

 rounded, the posterior border truncated ; umbones not prominent, nearly straight, costae 

 few, tuberculated, disposed in a series of rows which anteriorly are slightly curved, passing 

 obliquely downwards, posteriorly the costse are larger, and are curved upwards at a con- 

 siderable angle ; the area is flattened, transversely striated in the young state, but nearly 

 smooth in the adidt ; the inner carina is slightly tuberculated, and the tubercles upon the 

 marginal carina are more distinct. 



The series of costae posteriorly, which at first are tuberculated and moderately curved, 

 afterwards gradually become large, irregular compressed varices, which are directed nearly 

 perpendicularly downwards, and form a considerable angle at their junction with the 

 posterior portions of the costae. The inner carina is small and indistinctly tuberculated; 

 the marginal carina is much larger, but has likewise indistinct tubercles, which disappear 

 altogether in the adidt state of growth ; the area is divided into two parts by a slight 

 longitudinal furrow. 



The examples of this species are moderately numerous and of every stage of growth, 

 so that ample materials are afforded for comparison. The surface markings underwent 

 a continuous change throughout the life of the Mollusk; in the earliest condition, when 

 the length is only 6 or 7 lines, the surface has a few regular curved and smooth costse, 

 which form an angle or prominence as they pass over the marginal border (or position of 

 the carina), to the area which they cross, forming so many large plications ; when about 

 seven costae have been perfected, those which succeed begin to have their posterior 

 extremities more curved and indented to form tubercles, the plications upon the area have 

 then degenerated into striatums ; ultimately these latter become indistinct, and the 

 portion of the area last formed is nearly smooth ; the posterior extremities of the costae 

 gradually become large varices, which are directed nearly perpendicularly downwards, 

 and are imperfectly united to the anterior portions. The minute shell from Ancliff, 

 figured in the ' Mineral Conchology' under the name of Trigonia cuspidata, Sow., is 

 probably the present species in its earliest stage of growth. 



It will also be perceived, that the young shell very nearly resembles the same stage 

 of Trigonia Moretoni the only distinction residing in the more prominent costae of T. 

 Goldfussii. The Great Oolite shells never attain to the magnitude of those figured by 



