52 SUPPLEMENT TO GREAT OOLITE MOLLUSCA. 



testacea, which are believed to represent estuary conditions, a geological horizon which 

 possibly is not very dissimilar to that of our specimen. 



Trigonia arata, Lye. Tab. XL, fig. 2. 



Testa ovato-trigona, suhdepressa, umbonibus antemedianis, obtusis depressis, latere 

 antico brevi, rotundo, postico oblique declivi, area planata, oblique irregulariter , striatis, 

 carina marginali subnullo, lateribus costis antice obliquis, postice angulatis, depressis, sim- 

 plicibtis. Testa atate juniori costis concentricis simplicibus. 



Shell ovately trigonal, somewhat depressed ; umbones anterior to the middle of the 

 valves, obtuse and depressed ; anterior side short, rounded ; posterior side sloping obliquely 

 down ; area flattened, with transverse, irregular striations ; marginal carinae not conspicuous, 

 and obsolete posteriorly ; the sides of the valves with numerous closely arranged, obliqne, 

 plain costae, which are bent upwards posteriorly at an obtuse angle, and meet the area at 

 a right angle. 



The costse are rounded and rather depressed ; they become more distantly arranged 

 posteriorly, but only slightly increase in size. The young shell has the costse regular 

 and concentric ; the marginal carina is small, but forms a distinct elevation ; a specimen 

 more aged than the one selected for our figure has the posterior portions of the costae 

 slightly nodulous ; anteriorly they become waved and irregular. 



Geological Positions and Localities. A rare species ; Mr. Walton's specimens are 

 from the Forest Marble of Earleigh, near Bath ; it has also occurred in the same position 

 near to Cirencester. 



Trigonia Bathonica, Lgc. Tab. XL, fig. 3. 



Testa subtrigona, depressa, umbonibus altis mediants, latere antico et postico subrecto, 

 oblique declivi, lateribus costis elevatis, angustis, crebris, subundulatis, et spinulosis, 

 oblique instructis ; area parva planata oblique striata, carina marginali minimo, subnullo. 



Shell subtrigonal, short, depressed ; umbones elevated, mesial, and not recurved ; anterior 

 and posterior borders nearly straight, sloping obliquely downwards, the surface with 

 numerous (about twenty-four) narrow, elevated, spinose, and somewhat undulated oblique 

 costae, Avhich are directed from the marginal carina anteally downwards, and all reach the 

 lower margin ; the area is narrow and obliquely striated ; the marginal carina is very small, 

 and rather indistinct. 



The narrow, ridge-like costae have numerous minute, obtuse spines, which impart 

 roughness to the surface ; they are distinct, rather irregular, and therefore very different 

 from the serrated, elevated, regular costae of T. striata, Miller, and its allied species ; the 



