120 INTRODUCTION. 



prolong its edges so as to reach the surface of the sea bed : in the locality in which this form 

 is found, a vast number of other species of the genus occur, which do not present this pecu- 

 liarity, so that we must regard the structure as normal, although M. d'Orbigny's explanation 

 of its function is probably correct. The thickness of the shell in many species is very 

 considerable ; but we must refer our reader for more ample details to the most valuable 

 monograph of this genus, published in 1847, by M. de Koninck, as well as to several 

 publications on the same subject, by Professor King, M. de Verneuil, and others, in 

 which a multitude of details will be found which my limited space will not admit of 

 mentioning here. Various authors have proposed to arrange the numerous species 

 belonging to this genus into several groups, chiefly founded on the manner in which the 

 shell was ornamented. 



Geol. range. — Productus does not seem to have had so extensive a vertical range 

 as either Strophomena or Orthis, being restricted to the Devonian, Carboniferous, and 

 Permian periods, and specifically rare in the first and last of these.^ 



Examples: P. giganteus, Martin, sp. ; latissimus, Sow.; striatus, Pischer, sp. ; cora, 

 D'Orb. ; Erminius, De Kon. ; Medusa, De Kon. ; semireticulatus, Martin, sp. ; costatus, 

 J. Sow. ; Flemingii, Sow. ; spinulosus, Sow. ; scabriculus, Martin, sp. ; jimhriatus. Sow. ; 

 horridus, J. Sow.; tessellatus, De Kon.; undatus, J)eL; proboscideus, De Vern. ; hemi- 

 spharictis, Kutorga ; porrectus, Kut., &c. &c. 



Family— CALCEOLID^. 



Animal unknown. Shell probably free, valves not articulated ; ventral valve pyramidal, 

 with a large, flat, triangular area ; dorsal valve fiat, semicircular, with a straight hinge-line, 

 a small cardinal process, and two lateral groups of small apophysary (?) ridges ; no foramen 

 or muscular or vascular impressions. 



Obs. Only one genus is known, and so anomalous in its characters that its true place 

 in the class is far from being definitely settled. 



Genus — Calceola, Lamarck, 1809.^ 



Type—C. SANDALINA, Linn., Gmel., sp., p. 3349. Int., PI. IX, figs. 294—298. 



Anomia (part), of Gmelin, &c. 



Sandaliolithes, Schroter. 



CoNCHYTA JuLiACENsis, Crepites, Sandalites, Crepidiolithus, oi Hupsch. 



TuRBlNOLiiE, sp., Hisinger. 



Calceola, Lamarck, and of the generality of Authors. 



^ Some doubt seems still to exist if the genus was truly represented in the Silurian period. 

 Professor King believes, from the dispositions of the vascular system, that my so-called P. Twmnleyi 

 belongs to the genus Leptcena. 



2 Syst^me des Anim. sans Vert., p. 139 ; Hist, des Anim. sans Vert., vol. vi, p. 234, 1819. 



