120 

 TRIGONIA VOLTZII, Agass., plate iv., fig. 1. 



MONO. BEIT. Foss., TmaoNLZB, Lyc. y Pal. Soc., p. 20, plate x., fig8. 1-2. 

 Damon's Geology of Weymouth, Sup., pi. xv., fig. 2, 

 1880. 



This Kimmeridge Clay fossil lias often been confounded 

 with T. clavellata. It is larger, however, and considerably more 

 lengthened ; the umbones are somewhat more elevated, and 

 attenuated ; the anterior side is short, while the posterior is much 

 produced ; the test is also unusually thick. The valves have very 

 little convexity, consequently the surface of the area is more 

 nearly on the same plane with the other portion of the valve ; the 

 rows of tuberculated costse upon the other portion of the valve 

 are invariably less numerous, and more widely separated than in 

 T. clavellata. 



Obs. This shell is frequently met with in the Kimmeridge 

 Clays, both at Weymouth and Kimmeridge. 



TRIGONIA CYMBA. Contejean, plate iv., fig. 4. 

 MONO. BEIT. Foss., TEIGONLE, Lyc. } Pal. Soc., p. 192, plate xxxviii., fig. 1. 



This species is remarkable for the considerable elongation of 

 the valves posteally, for the extremely slight curvature of the 

 rows of costse, and which are nearly horizontal ; for their incon- 

 spicuous tubercles, and lastly for the small development of the 

 ornamentation of the valves ; the umbones are large, elevated, 

 and nearly erect ; the anteal portion of the shell has consider- 

 able convexity ; the posteal and more lengthened portion is 

 depressed ; area narrow, bounded upon each side by a row of 

 minute tubercles over the anteal or umbonal half of its length ; 

 the posteal half of the area has only transverse rugae, which are 

 not strongly defined, it is also much depressed; the other por- 

 tion of the shell has rows of clavellated costse about fifteen or 

 sixteen in number, small, nearly horizortal or coinciding in their 

 direction with the lines of growth. 



