CLAVELLATA. 21 



comparison of the two forms than to state that the figure of T. Voltzii will readily be 

 distinguished from the other by its greater length. 



Having, after great delay, obtained perfect specimens of the Kimmeridge Clay T. 

 clavettata (so called), I am enabled to affirm its specific distinctiveness from the typical 

 or Calcareous Grit examples of T. clavellata, and to give its distinctive features with 

 sufficient precision, figures of which will be given upon Plate X. Compared with that 

 form, T. Voltzii is a larger, and also, in proportion, a more lengthened form, the umbones 

 are somewhat more elevated and attenuated ; the anterior side is short, but the posterior 

 side is more produced ; the test is also unusually thick ; the convexity of the valves is 

 somewhat less, consequently the surface of the area is more nearly upon the same plane 

 with the other portion of the valve ; the area in its other features offers but little that is 

 distinctive, excepting that the transverse plications are unusually large, irregular, rugose, 

 and wrinkled ; they are united to the tubercles of the carinas. The rows of tuberculated 

 costae upon the other portion of the valve are invariably less numerous, and much more 

 widely separated, than in T. clavellata ; adult examples of the latter shell have sixteen or 

 seventeen rows ; the larger, T. Voltzii, has only eleven or twelve rows ; the tubercles also 

 have some differences, their number in each row is nearly similar, but the Kimmeridge 

 Clay shell has its tubercles compressed, or cuspidated and pointed, and, unlike the other, 

 they are much impressed by the lines of growth, which are unusually large and con- 

 spicuous over the whole of the valve. The largest specimens have the anteal portions of 

 the costse attenuated and cord-like ; the largest tubercles are near to the posteal extremities 

 of the rows. 



It may be a question of dispute how far the above-stated distinctions are of 

 specific value, or what are the limits of variability possessed by each of these two clavel- 

 lated Trigoniae ; without discussing the arguments which may be adduced for their 

 distinctiveness or otherwise, it will be sufficient to remark that the peculiarities of each 

 form are observable upon all the specimens in the geological formation where they 

 occur, and are therefore of value in a stratigraphical point of view. 



Separate valves of T. Voltzii occur in the Kimmeridge Clay of the coast of Dorset- 

 shire, in the same formation at Wootton Basset, Wiltshire, and in Lincolnshire ; examples 

 with the valves united are rare. The localities given by Agassiz are Argentenay (Yonne), 

 also Besancon ; Oppel gives Boulogne : all in the Kimmeridge Clay. 



