UNDULATiE. 69 



Stratigrapldcal position and Localities. Near to Stroud and Nailsworth, Gloucester- 

 shire, in the middle portion of the Inferior Oolite; in a bed of pale, tough, cream-coloured 

 limestone (Coralline mud), associated with Trigonia costatula, Lye, T. angulata, Sow., 

 T. PJnlKpsi, Mor. and Lye, a crowd of small, sub-cylindrical Nerinaeae, sub-acicular 

 Chemnitzise, and a numerous group of Molluscan forms, both of Gasteropoda and 

 Conchifera : but all the Trigoniae are rare. 



Trigonia geographica, Ag. Plate X, fig. 6. 



Trigonia geographica, Agassiz. Mem. sur les Trigonies, 1840, p. 25, tab. 10, 



fig. 7 (Excl. tab. 6, figs. 2, 3). 



— — IfOrhigny. Prodr. de Paleont., 1850, vol. ii, p. 17, No. 



267. 



— — Be Loriol, E. Royer, and H. Tombeck. Descr. Geol. et 

 • Paleont, des Etages Jurassiques Superieurs de 



la Haut Marne, 18/2. Mem. Soc. Linn, de 

 Normandie, torn. 13, pi. 17, fig. 7. 



Shell ovately trigonal, moderately convex ; umbones submesial, not much produced, 

 and only shghtly recurved ; anterior side produced, its border curved elliptically with the 

 lower border ; hinge-border straight, sloping obliquely downwards. Area narrow, somewhat 

 concave, divided into two portions by a considerable depression of the upper half; the 

 whole is transversely striated ; the bounding carinas are very small, without any distinct 

 tubercles. Escutcheon narrow, depressed, and moderately lengthened. The other portion 

 of the shell has numerous rows of narrow, plain, closely arranged costae, all of which 

 originate at the anterior border and pass over the middle of the shell obliquely downwards 

 with a slight curvature ; they enlarge somewhat posteally, and curve upwards towards the 

 carina ; about twelve or thirteen of the costae first-formed are plain, those which follow 

 gradually become tuberculated and enlarge at their posteal portions ; each succeeding row 

 becomes more tuberculated, so that the rows last-formed have only a short portion plain 

 anteally ; they thus gradually form two series of costae, of which the posteal or tuberculated 

 series is the larger and less numerous in consequence of the intercalation of three or four 

 rows of short anteal costae, which impart much irregularity to the few last-formed rows ; 

 the tubercles are neither regular nor symmetrical, some are distinct and oval, others are 

 united in the rows which all curve upwards to the carina. 



Our specimen resembles only the second figure of Agassiz ('Trigonies,' tab. 10, 

 fig. 7), but is a smaller example with less prominent posteal tubercles; the area is also 

 nearly destitute of ornamentation, which is probably due to defective preservation. The 

 figure given by De Loriol, Royer, and Tombeck is more lengthened, and the costae have 



10 



