﻿30 BRITISH FOSSIL TRIGONIiE. 



they have the tubercles small, separate, rounded, regular, and nearly of equal size, 

 excepting near to the anterior border, where the costse are attenuated and their tubercles 

 small and cord-like or indistinct ; the last-formed six or seven rows pass upwards nearly 

 perpendicularly to the marginal carina, with which they form a considerable angle ; these 

 portions of the last-formed costte are cord-like or imperfectly tubei'culated ; there are 

 thirteen or fourteen tubercles in each row. 



Most commonly the rows of tubercles are symmetrical and continuous across the entire 

 valve, but occasionally the anteal portions of the valves have the tubercles confused and 

 irregular; in such instances the tubercles continue rounded and separate. 



The examples upon our plate, which are only of medium size, indicate that the species 

 possessed variability both in the general figure and in the ornamentation. Some speci- 

 mens obtained near Chipping Norton are remarkable for the raised ridge-like figure of 

 all the costse, the indistinctness of their tubercles, the coarseness of the plications across 

 the area, the great convexity of the valves, and the recurvature of the umbones. A speci- 

 men with the valves spread open, now in the National Museum, Jermyn Street, is 

 remarkable for these peculiarities. The largest example I have seen is in the collection 

 of Professor Buckman, of Bradford Abbas ; its length is four and three quarter inches, 

 and is from the Upper Trigonia-grit of Rodborough Hill ; the area has coarse plications 

 which render the carinte obscure. The figure given by Messrs. Dewalque and Chapuis is 

 very distinctive in the characters of its costse, but the three large cord-like carinse upon 

 the area differ altogether from the numerous examples that have been brought under my 

 notice. 



The very indifferent examples figured by Agassiz, especially that of his Plate III, fig. 8, 

 induced me at first to regard the British forms as a distinct species. The attenuations of 

 the carinal extremities of the costse, and their increase of size towards the anterior and 

 the lower border, as depicted in the example above quoted, are altogether unlike British 

 specimens, from whatever locahty they may be obtained ; it is, therefore, just possible 

 that the first figure of Agassiz may really represent another species, even if we allow 

 some latitude for variability in the ornamentation. 



Additional examples of T. clavo-costata, Lye, indicate that it is the immature condition 

 of a large variety of T. sit/nafa, in which both the tubercles upon the costse and those 

 also upon the marginal carina participate in general increase of the dimensions ; the figure 

 is also less lengthened posteally than in the typical form. The few specimens which I 

 have examined are from white limestone in the vicinity of Stroud. 



Affinities and differences. Prom T. clavellata. Sow., and T. per lata, kg., T.si^nata is 

 sufficiently separated by the more depressed form, the more numerous rows of costce, the 

 smaller and more numerous tubercles, together with the considerable angle at which they 

 approacli the marginal carina : others of the ClavellaicB are more remotely allied. 



Stratigraphical position and localities. T. signata ap[)ears to be limited to the Inferior 

 Oolite, in which it has occurred at numerous localities, both British and Continental, but 



