34 BRITISH FOSSIL TRIGONLE. 



nised ; the peculiar imbrication of the costse noticed by Mr. Sowerby appears to arise 

 from the erosion of their rounded tubercles. 



In the ' Mineral Conchology ' it is described as " transversely oblong, depressed, with 

 five or six concentric, dentated, sub-imbricated keels upon the rounded anterior side ; the 

 posterior side obliquely truncated, ribbed. The carinse upon the surface of this little 

 shell resemble terraces, one above the other ; each is divided into four or five angular 

 lobes/' 



The little specimen herewith figured is larger than the type in the ' Mineral Concho- 

 logy ;' it has seven rows of regular, concentric, tuberculated costse, each of which has five 

 or six distinct tubercles j anteally the rows become attenuated and only slightly tubercu- 

 lated ; posteally they are well separated from the marginal carina, which consists of a row 

 of somewhat smaller tubercles, corresponding in number to the rows of costae; the area 

 has transverse, plain costellse, each of which is united to one of the carinal tubercles. 

 Our specimen is slightly broken posteally. 



The few minute specimens hitherto examined differ from the young condition of all 

 the known Clavellated Trigonias of the Lower Oolites, and are believed to constitute a 

 distinct species. 



Geological position and localities. Ancliff and Bath in the shelly Great Oolite. 



Trigonia Griesbachi, Lye. Plate III, fig. 10, a, b. 



Tbigonia tuberculosa, Lycett. Pal. Soc. Suppl. Monog., 1863, p. 47, pi. xl, fig. G; 



uot T. tuberculosa, Lye, Ann. and Mag. Nat. 

 Hist., 1850, t. ii, fig. 9. 



The little T. Griesbachi is only known by a single specimen, which fortunately is in 

 so excellent a condition of preservation that its entire specific characters are fully exposed, 

 and have been faithfully delineated in the magnified figure above cited, and published by 

 the Palseontographical Society in 1863. At that period a single specimen of T. tuber- 

 culosa was all that remained at my disposal for comparison, and its condition was by no 

 means in so satisfactory a state ; it was, without doubt, nearly allied to the Cornbrash 

 Shell, and making some allowance for difference of mineral character, and of geological 

 position, I was induced to regard the two as not specifically distinct or differing only 

 within the limits that might possibly be induced by altered conditions of geological 

 habitat and of fossilisation. The examination of additional specimens of the Inferior 

 Oolite shell have convinced me of the real distinctness of the two little Trigonias, and 

 that their distinctive characters are as follows : 



T. Griesbachi has nearly the general outline of its ally, but is more depressed, so that 



