40 SUPPLEMENT TO GREAT OOLITE MOLLUSCA. 
surface, more especially towards the umbones, 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 umbonal extremity a 
thickness of two inches and a half, the cavity of the imterior 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 Lithophagide, that, unlike the common Lime 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 Lime. 
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 rugz and the tubular, 
spine-like processes ; it is, in truth, aa equivalve Spondylus, destitute of hinge-teeth. The 
variations of figure are also considerable ; sometimes sub-orbicular, with no more obliquity 
than a Pecten or Spondylns, with the sides nearly equal, the radiating coste 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 Z. sguammicosta, 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 inaequivalve, 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 Lime, 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 Lime which are 
nearly allied, both in their external characters, shell structure, and mode of growth ; these 
