ORTHID^. 271 



four, so long the lateral ones are seven. As soon as there is even an appearance of a 

 departure from this number on the mesial lobe and sinus, and where the rudiment of an 

 additional plait is visible, we then find the lateral plaits to be nine or ten. These changes 

 take place independent of the size of the shell or of its rotundity ; all variations in the 

 plaits of the sinus being accompanied by a change in the number of lateral plaits. 

 This is illustrated in the figures of nearly equal dimensions, where the specimen fig. 1 a 

 has three plaits on the sinus and four on the mesial lobe, and seven on each side ; while 

 fig. 1 G has a partial development of the fourth plait in the sinus, and an obscure rudi- 

 ment of a fifth on the mesial lobe, and at the same time it has ten on each side of the sinus. 

 Even irregularities in the development of the mesial plications are followed by irregula- 

 rities in the lateral ones. A single specimen has two plications in the sinus and three on 

 the mesial lobe, and also on one side of the slo[)e of the sinus, as well as on the slope of 

 the mesial lobe, a single plication, which is intermediate to the mesial or lateral series, 

 and the lateral plaits, in this case, are six on one side and seven on the other." 



It is, therefore, very difficult to admit varieties foimded upon the number of ribs, and 

 we have elsewhere shown how the number of plications varies in a similar manner in 

 many forms, and especially in Bhynchonella. 



M. de Verneuil has proposed to divide the Russian forms of the shell under descrip- 

 tion into several named varieties, and adds, with reference to Schlotheim's Terebratulites 

 biforatus, which he maintains as the typical form of the species, that the description given 

 by Schlotheim of his T. biforatus is so short that it might equally Avell apply to 

 the three varieties which De Verneuil distinguishes, namely, var. Lynx, var. dentatus, and 

 vai'. chaiiia. But Von Buch, having seen in the Berlin Museum the authentic specimen 

 upon which Schlotheim's species was established, explained to De Verneuil the 

 diff'erences which distinguish it from its principal varieties ; these distinctive characters 

 being only the presence of five ribs in the sinus, and a width proportionately greater. 

 Nevertheless, adds De Verneuil, 0. biforatus is provided, as are all the forms of the same 

 group, with two areas and two triangular open fissures ; the ventral [our dorsal] valve is 

 also thicker than the dorsal [our ventral] one, and the number of the lateral ribs is 

 about the same as in the var. Lt/nx, a variety close to the type, and in which one 

 may commonly count eight or ten ribs on either side of the sinus. 



Our British specimens seem to vary to so great an extent that I would scarcely know 

 where to draw a line of demarcation between them. We certainly have the type biforata 

 and the var. Li/)ix, but these two seem with us so intimately connected that I have 

 combined them under Schlotheim's single designation ; they are represented by figs. 

 11, 12, 13, and 14 of Plate XXXVIII. 



We have, also, perhaps a well marked variety to which M'Coy has given the name of 



fissicostata (fig. 19, &c.), which that author describes in the following manner: — • 



" General characters of the preceding varieties [Sp. biforata, var. Lynx, var. dentata). Four 



ribs on the rostral part of the mesial furrow, the two outer of which usually branch at 



