﻿CLAVE LLATiE. 37 



striations are cut oif abruptly at tliat border. The marginal carina is always clearly 

 defined ; it is very narrow, ridge-like, minutely tuberoulated, and has only a slight 

 curvature. 



The escutcheon is narrovt^, lengthened, and much de|)ressed ; its superior border is 

 considerably raised. The other portion of the surface has about twenty-two narrow, 

 obliquely curved, and elevated costse; they are small towards the carina, which they 

 touch, and descend almost perpendicularly ere they curve towards the anterior border. 

 The last-formed six or seven costse attain tlie lower l)order ; each costa is fringed with a 

 densely-arranged row of small, elevated, obtuse tubercles, which are frequently somewhat 

 compressed laterally. The interiors of the valves have the lower border prominently 

 dentated. 



The most remarkable features in this species are the short sub-quadrate figure and the 

 large size of the area; the height is equal to, or even slightly exceeds, the length, and the 

 surface of the area is equal to two fifths of the entire valve. Its general aspect is so 

 peculiar that it will not readily be mistaken for any other species ; but the only authentic 

 figures are those in the ' Mineral Conchology,' which can only be described as bad 

 specimens badly drawn, and this will account for the fact that both Goldfuss and Agassiz 

 have fallen into error respecting it; each of them has figured for T. striata a different 

 species. 



Another allied species is T. spimlosa, described but not figured by Messrs. Young and 

 Bird in their ' Geology of the Yorkshire Coast,' and soon afterwards figured but not 

 described by Professor Phillips, in his ' Geology of Yorkshire,' ;is T. striata, and subse- 

 quently by Agassiz as T. tuberculatfi. In this species the much more lengthened area and 

 the different costae with their larger tubercles will serve to distinguish it. The name 

 chosen by Miller refers to the transverse striations upon the area. The little shell figured 

 by Agassiz for T. striata. Miller ('Trigonies/ tab. 4, fig. 12), is not that species but the 

 single small figure, and insufficient description renders it difficult to be assigned to any one 

 of the allied Inferior Oolite forms. The characters of the rows of tuberculated costae 

 differ equally from T. Moutierensis, T.formosa, and T. Phillipsi ; the general figure has 

 no resemblance to T. striata. 



Straiigraphical position and localities. The zone of Ammonites Huraphriesianus in 

 the Inferior Oolite at various localities in the Counties of Somerset and Dorset. It appears 

 to be altogether absent in the more northern extension of the Inferior Oolite in its course 

 through the Cotteswold Hills, and is also absent in Oxfordshire, Northamptonshire, Lin- 

 colnshire, and Yorkshire ; it appears, therefore, that the Mendip Hills presented a dividing 

 barrier at the period of the deposition of the Inferior Oolite formation, and thus had an 

 important influence upon the distribution of its testaceous mollusca. 



