64 Transactions. 
of those shells; but it seems more probable that the separation took place 
at an earlier stage than that of G. selwynensis. 
The Tertiary species fall into three well-defined groups :— 
I. The laticostata group, characterized by— 
(a.) Raised rounded ribs (a few exceptions). 
(b.) An equilateral outline, circular to dorso-ventrally oval. 
(c.) Closely set ligamental grooves on a highly inclined area. 
(d.) Valves little inflated, beaks low (exceptions). 
(е.) Teeth small and numerous until invaded by the area. 
П. The huttoni group, characterized by— 
a.) Linear interstices between low, flat ribs (exceptions). 
(b.) An inequilateral outline, subquadrate to obliquely ovate. 
(с.) Well-spaced ligamental grooves on a moderately inclined 
area. 
(d.) Valves inflated, beaks prominent. 
(e.) Teeth large and strong. 
ome specimens of this group present an external resemblance 
to Cucullaea, but they can always be recognized by their rounded 
shoulders. 
ПІ. The Axinea group, characterized by— 
(a.) Almost smooth surface, under a lens showing very fine 
radial riblets over the whole disc, superimposed upon 
a system of narrow obsolete ribs corresponding to the 
fine crenulations on the inner ma gin. Sometimes these 
ribs are shown only on weathering. 
(b.) Outline circular to ovate. 
(c.) Many closely set ligamental grooves on a highly inclined 
area. 
(d.) Valves inflated, beaks moderate. 
(e.) Teeth numerous and fine. 
The fine secondary riblets so well developed in this group 
often appear in other groups, but then only on a small portion 
of the disc. 
This group corresponds to Azxinea of Poli, 1791, used as à 
separate genus by Cossmann, Angas, and others; as a section 
of Glycymeris by Fischer and Adams, and as a synonym of it 
by Zittel, Dall, and Suter. 
The three divisions of the New Zealand Species are probably of sub- 
generic value, the first being Glycymeris s. str., the second a new subgenus, 
and the third Azinea. The definite classification into subgenera has not 
been carried out in this paper, as no specimens from abroad were available 
for comparison. E 
DESCRIPTION oF SPECIES. 
of the shell. The ribs on the latter are narrower and more rounded, and 
the interstices are wider than those of the former. 
