18 MOLLUSCA FROM THE GREAT OOLITE. 



PTEROPERNA COSTATULA, Deslongchamps, sp. Tab. II, figs. 8, 8a, 13, 13a. 



GEKVILLIA COSTATULA, Deslongchampg. Mem. Soc. Linn, du. Calvados, 1824 ; torn, i, 



t. v, figs. 35. 

 AVICULA POLYODON? Buviffnier. M&n. Soc. Philomath. Verdun, 1845, pi. iv, fig. 16. 



Testa obliqud, lined car dinali recto, elongato, postico valde producto et emarginato, valvd 

 sinistrd modico convexo, valvd dexlrd subpland, unbone depresso, latere postico in utraque 

 valvd curvato aut excavato. 



In (Btate juniori, valvd sinistrd convexo-brevi, costulis radiantibus (6 8) elatis, acutis 

 subundulatis, et iruequalibus cum lineis transversis interstitialibus regularibus et tenuis- 

 simis. 



In estate adulto, valvd sinistrd sine costulis aut striis cum laminis incrementi paucis, 

 distantibus. Valvd dextrd semper l&vigatd. 



Shell oblique, hinge line straight, elongated, very much produced posteriorly and 

 emarginated, left valve moderately convex, right valve more flattened, the umbo depressed; 

 posterior side in each valve curved, its margin concave. 



This species occurs under the following conditions of growth : 



In the young state, the left valve is very convex and short, having radiating costae 

 (6 8) elevated, acute, slightly waved and unequal, the interstitial spaces with regular, 

 transverse, fine, closely arranged lines. 



In the adult state, the left valve is without costae or striae, having only a few distant lines 



of growth. Were instances wanting to exemplify the advantage which is derived from the 



inspection of a large number of specimens in every stage of growth, undoubtedly the 



present species might be selected for such a purpose, the two extremes of growth presenting 



an aspect so dissimilar, that until numerous examples of every intermediate grade had been 



obtained, we hesitated with respect to their specific distinctness or identity ; minute 



specimens occur in great numbers, having a length of only three or four lines ; in these 



the costae are always very prominent, the number of costae vary from 6 to 8, they occupy 



only the middle portion of the valve, the sides being plain. The costae continue 



distinct, but less conspicuous, when the shell has attained a diameter of 16 or 18 lines, 



but the costae have then become waved, irregular, and unequal; beyond these 



dimensions, the valve is either plain, or has only faint indications of costse, crossed and 



interrupted by laminae of growth ; but even in the ultimate stage of growth, when the 



hinge line has attained the length of five inches or upwards, and the test has acquired a 



considerable degree of thickness, the left valve has never so smooth a surface as the 



other, the last faint indication of its having previously possessed a sculptured surface. 



The specimens figured by M. Deslongchamps and M. Buvignier have only a very remote 



resemblance to each other, and tend to illustrate the foregoing remarks ; Gervillia 



coslatula is stated by the former author to have only four or five costae, but the smaller 



number is probably owing to the less perfect state of the specimen, or to accident ; in the 



