ARCA. 175 



of sufficient information on this point, it is at present 

 impossible to make a complete comparison between the 

 so-called species. All the fossil specimens which I have 

 seen from the Christiania and Uddevalla districts be- 

 long to A. glacialis, and they significantly indicate the 

 climatal conditions which prevailed during the period 

 immediately preceding the elevation of these sea-beds. 



A. pectunculoides, being found in the Coralline Crag, 

 as well as in the upper tertiaries of Belgium and Sicily, 

 would appear to be the older of the two. Although I 

 am not aware of any intervening form having been dis- 

 covered, such may have existed ; and supposing that to 

 be the case, it would be fair to infer that A. pectuncu- 

 loides was the ancestor of A. glacialis. Naturalists 

 have been so much accustomed to regard species in an 

 objective point of view, and not as abstract ideas, that 

 it is difficult to bring their minds into the proper frame 

 of thought for discussing speculative theories upon con- 

 fessedly so difficult a question as the origin of species. 



The present species is the A. raridentata of Searles 

 Wood, who has recognized, in his work on the Crag 

 Mollusca, the priority of Scacchi's publication, and 

 adopted the name which I have now given ; and it ap- 

 pears to be also the A. pusilla of Nyst. 



B. Shell equivalve : teeth numerous and uniform, set across 



the hinge-plate, and either divided into two rows or 

 arranged in a single and continuous row. 



2. A. obli'qua*, Philippi. 



A. obliqua, Phil. Faun. Moll. Sic. ii. p. 43, t. xr. f. 2. 



Shell obliquely oval, with a rhomboidal outline, much nar- 

 rower at the anterior side, and spreading out on the other side, 



* Oblique. 



