40 SUPPLEMENT TO GREAT OOLITE MOLLUSCA. 



surface, more especially towards the urnbones, the triangular sub-umbonal area is large 

 and oblique, so that the umbones then are widely divergent ; the internal radiating flutings 

 have gradually disappeared, or are only visible at the lower border of the valves ; the mus- 

 cular scar is conspicuous ; ultimately, each valve acquired at its utnbonal extremity a 

 thickness of two inches and a half, the cavity of the interior became much smaller, the 

 outer surface ceased to be extended at its borders during this internal accretion of shell ; 

 we may also infer, from its solid, ponderous mass, and from the frequency with which it 

 became perforated by the Lithophagidas, that, unlike the common Limaa and Pectens, its 

 habits were sedentary ; doubtless these perforations may have been made in dead shells, 

 but they are not to be discovered in any other of the Jurassic Lima?. 



The test consists of two very distinct layers ; the outer layer is always thin and semi- 

 transparent, the inner layer is white, opaque, laminated, and received continual additions 

 to its thickness ; in brief, the structure and mode of growth agrees with that of the genus 

 Spondylus as fully as does the external aspect of the imbricated rugae and the tubular, 

 spine-like processes ; it is, in truth, an equivalve Spondylm, destitute of binge-teeth. The 

 variations of figure are also considerable ; sometimes sub-orbicular, with no more obliquity 

 than a Pecten or Spondylus, with the sides nearly equal, the radiating costse undulating 

 and irregular, as in Hinnites ; in other instances it is oblique, with a steep anterior slope ; 

 add to this latter figure a greater lengthening of the valves, a compression of the posterior 

 side, and the aspect becomes strictly that of Lima, as in L. squainmicosta, Buv., which 

 appears to be only the young condition of this variety. 



Few shells differ more in the convexity of the valves ; occasionally an example will be 

 found so much inflated that its figure can only have resulted from having been moulded 

 upon and remained closely adherent to a convex surface, to which the missing valve 

 probably remained attached. 



The shell is not irisequivalve, although such an appearance is often imparted to it 

 from a depression, or an irregularity in the convexity of one of the valves ; as, however, 

 the borders of the valves are found to fit perfectly, this distortion cannot be owing to the 

 effects of fossilization. 



Even from the earlier days of paleontology this shell has been a source of 

 doubt and perplexity. Schlotheim referred it to Ostracites, as also did Ziethen. 

 Mr. Sowerby, in the ' Mineral Conchology,' placed it with Lima, but expressed doubts as 

 to the genus; more recently, Professor Quenstedt, in his 'Jura,' after alluding to the 

 features which distinguish it from the ordinary Limse, divides it into two varieties, one 

 having a thick and the other a thin shell; he concludes by assigning it to the genus 

 Ostrea, but without offering any proofs that it would be correctly placed with the latter 

 genus. The change from the thin to the thick shell has already been explained, and the 

 structure of the test is distinct from that of Ostrea. 



Lima pectiniformis may be placed at the head of a group of Jurassic Lima? which are 

 nearly allied, both in their external characters, shell structure, and mode of growth ; these 



