78 BRITISH FOSSIL TRIGONI^. 



middle of the valve form a sudden curvature or an imperfect angle ; their posteal 

 portions curve upwards towards the marginal carina, which they meet at a considerable 

 angle. The rows are nearly equal in size throughout their length, but become somewhat 

 attenuated towards their two extremities. Some specimens have the costse almost 

 entirely plain, or only sparingly and slightly knotted (PL XVI, tig. II), others have the 

 tubercles small, numerous, and irregular ; usually the anteal portions of such have the 

 tubercles more or less lamellated, their posteal portions more frequently form rounded 

 tubercles ; in other instances they are only sub-tuberculated or cord-like (PI. XVI, 

 fig. 9 ; PI. XVII, fig. 5) ; each row extends to the marginal carina, the tubercles of 

 which form the terminations of the rows of costse, but towards the middle and lower 

 portion of the carina its tubercles become somewhat compressed, and are more numerous 

 than the rows of costae. An examination of numerous specimens in various conditions 

 of preservation and in different stages of growth prove that the costae have great 

 variability, both in their figure and in the presence or absence of tubercles. Occasionally 

 a specimen will occur with the costae plain and oblique, forming a considerable angle with 

 the posteal portion, which, over the lower half of the valve, rise upwards almost at a 

 right angle with their anteal portions, but even such specimens are never altogether 

 destitute of small depressed nodose elevations upon the costse ; others are irregularly 

 tuberculated or nodose over two thirds of the height of the valve, and are more or less 

 angulated ; again, other specimens have the few last-formed costse altogether variable 

 both in their direction and in the size and arrangement of their nodose tubercles which 

 meander across the valve (PI. XVI, figs. 9, 10), but their posteal portions are invariably 

 nodose and curve suddenly upwards to the carina. Without exception fully developed 

 forms have one or occasionally two supplementary anteal costae. 



The example figured under the name of T. arata in the ' Supplementary Monograph 

 of the Great Oolite MoUusca,' above cited, was one of several very indifferently preserved 

 specimens obtained by Mr. Walton in the Forest Marble of Farleigh, near Bath; the 

 specimen is not of fully developed growth, the costse are plain or have only slight 

 indications of tubercles, and there is no distinct carina ; other examples forwarded to me 

 by the same gentleman were of more advanced growth and tuberculated, but so im- 

 perfect as to be unfit for purposes of illustration. The portions of their surface preserved 

 agree with the larger and more irregular of our Lincolnshire examples. 



Affinities and differences. From T. v.-costaia it differs in the figures of the costae, 

 which are not V-like ; the area is also much more rugose, and it is without the median 

 carina. 



It is more nearly allied to some forms of that very variable species, T. Moretoni ; 

 unlike the latter, the posteal portions of the costae do not enlarge, but become somewhat 

 attenuated near to the carina, to which, in the best preserved examples, they are united, 

 each small tubercle upon the carina forming the terminal tubercle of one of the rows of 

 costse ; the median carina in T. Moretoni has no counterpart in T. uyiddata ; the latter- 



