10 



fossils is almost identical, but the sculpture of each is very distinct. In 

 Trigonia Vau, the ribs at the anterior end incline obliquely downwards 

 before they change their course ; in T. diversicostata their general direc- 

 tion is either longitudinal or upwards, but the most striking difference 

 is in the markings on the posterior area of the two shells. In T, diversi 

 costata that region is boldly and longitudinally ribbed ; in T. Vau it is 

 transversely striated and '• divided into two parts by a slight longitu- 

 dinal ridge." 



Trigonia. (Sp. undt.) 



Plate X., figure 2, 2a. 



Shell compressed, elongate, subtrapezit'orm, narrowest behind ; anterior 

 end xevy short, posterior produced ; length much greater than the height. 

 Beaks small, anterior, subterminal, slightl}' recurved, not much elevated 

 above the superior border. Lunule none ; posterior area flattened later- 

 ally, with a rounded margin, made up of two elongately subtriangular 

 spaces, one on each side of the hinge line ; ligament extei-nal, short, thick, 

 prominent and transversely striated. Hinge line straight, sloping gently 

 downwards ; posterior end obliquely subtruncate. Anterior end almost 

 straight, but curved a little outwards; antero-ventral margin broadly 

 rounded ; base line convex in the middle, straighter, and curving much 

 more gradually upwards behind. 



Out of nine specimens on which the test is partly preserved and a 

 number of imperfect casts, none shew the true characters of the whole of 

 the surface ornamentation. The oi'iginal of fig. 2, on Plate X., gives the 

 clearest idea of the normal shape of the shell, besides showing the sculp- 

 ture of the beaks and posterior area. Fig. 2a, on the same Plate, is a 

 repivsentation of the most poi-fect of four distorted individuals which 

 have been compressed laterally, and whose exterior is either worn, ex- 

 tolialed, or partly covered by the tenacious matrix. The only informa- 

 tion afforded as to the sculpture of the main body of the shell, is that 

 supplied by the four last mentioned examples. In these there appear to 

 be about thirteen or fourteen obliquely transverse, concavel}', curved 

 rows of separate raised tubercles. The whole of the rows commence at 

 the outer edge of the posterior area, and they all run obliquely down- 

 wards and forwui'ds. Eather more than half terminate at the anterior 

 margin, but just before reaching it they each turn abruptly upwards. 

 The upward liend at the fi-ont margin consists only of a change of 

 direction of the last tubercle, and there is a solitary intervening one, at 



