CARDIOMORPHA CORRUGATA. 273 



Observations. — M' Coy's species, C. corrugata, is very distinct, being sepa- 

 rated at once from all other species of the genus by its general shape and 

 ornament. It seems to attain a much larger size than any other, but with 

 advanced growth becomes more transversely oval and less orbicular. De Koninck 

 separated this species from Gardiomorpha, placing it in the genus Isoculia, which 

 name, it seems, M'Coy had proposed for the shell on which de Koninck founded 

 Gardiomorpha, being unaware of this fact. This name, however, only appeared 

 by mistake in the explanation of a plate, and was never really brought into use 

 by M'Coy. De Koninck bases his reason for separating G. corrugata from 

 Gardiomorpha on the strength of it possessing a lunule — an observation which I 

 believe to be erroneous. There is really no lunule marked out by definite lines ; 

 but, in common with all species of Gardiomorpha, G. corrugata has its anterior and 

 upper part compressed and hollowed without there being a definite lunule. The 

 hinge-line of the species under discussion is very much compressed and raised 

 in front, so as to stand erect (see figs. 5 and 6, PI. XXIII) ; and, though 

 the sulci and ridges are absent lower down, there is no definite lunule marked 

 off by a distinct margin from the rest of the upper surface of the valve. 

 If G. corrugata has a lunule, all other species of the genus have exactly the same 

 character quite as well developed. De Koninck has mistaken the absence of 

 ridges and sulci at the extreme antero-superior angle, and taken the resulting 

 smoothness as evidence of a lunule. He says (op. cit., p. 17), "Isoculia . . . 

 se distingue non seulement par les fortes rides concentriques, mais encore par la 

 lunule qu'elle posede au-dessous de ses crochets. . . . C'est principalement ce 

 dernier caractere qui sert a separer le genre Isoculia du genre Gardiomorpha, dont 

 les especes sont completement depourvues. J'ajouterai encore que les crochets de 

 ces dernieres especes sont beaucoup moins enroules et moins r profondement situes 

 que ceux des especes appartenant au genre voisin." The umbones are, perhaps, 

 somewhat less enrolled than those of G. oblonga, but are more so than in G. parva. 

 G. ventricosa possesses well-marked concentric grooves ; and I therefore see no 

 ground for retaining de Koninck's genus Isoculia, under which he described 

 only two forms. " Isoculia undata," he says, " a les plus grands rapports avec 

 risocula corrugata, F. M'Coy, et je l'eusse considered comme ne formant que le 

 jeune age de celle-ci, si le paleontologiste Irlandais n'avait fait remarquer, et si 

 moi-meme je n'avais pu constater qu'en cet etat sa forme etait generalement sub- 

 circulaire, tandis que celle de VIsoculia undata est ovale. C'est principalement 

 dans cette difference de forme que reside son caractere distinctif." Both these 

 species occur together at the same horizon in Belgium, and I think it is probable 

 that a large suite of specimens would show that this form is merely a variety of 

 M'Coy's species. 



De Koninck says that G. corrugata is limited to the middle or Waulsortian 



35 



