G 
Tf.-DESCRIPTION OF TIIF GENFFA AND SPECIES. 
C/as.s— PELECYPODA. 
On/er—PRIONODESMACEA. 
Section — IS030NTA. 
AVICULOPECTINID^., 
Genus — AYICULOrECTEN, McCoy, 1851. 
(Ann. ilag. Nat. Hist., 1851, YU (2), p. 171.) 
Observations. — )McCoy’s definition of liis geims is a very clear and 
conipreliensivc one, the salient points being an inequivalve, inequilateral 
shell, more or less produced towards the posterior, the hinge-plate consisting of 
a narrow facet along the dorsal margin supporting tl'.e “ cartilage ” (resilium), 
and devoid of a “ ligamentary ” pit (chondrophore) ; in other wmrds, in the 
presence of the one feature and absence of the other lies the principal 
ditfercnce between Pecten and Aviculoyeeten . 
Some Authors have departed widely from this very clear and precise 
definition — practically ignoring it. Thus the late Professor James Hall 
emended the genus to ‘•'include those forms whicli have the hinge-line 
shorter than the transverse diameter, and both ears well detined.” ^ lie also 
proposed Lyrioyeclen for those .shells with a “ short hinge-line and very small 
anterior ear,” and Pterinopectefi to include those with “ears not well detined, 
being simple expansions of the upj)er lateral margins of the hinge-line.” As 
a matter of fact, ditfercnccs of this Jiaturc are purely empirical, and inorc 
of a specific than generic nature; Ave consider both the above as simple 
synonyms of Aviculopecten. Equally unphilosophical is Dr. W. IVaagcn’s 
definition of Aviculopecten. lie says : “ I have, as a rule, considered as 
belonging to the genus Aviculopecten those species having the anterior Aving 
smaller than the posterior one; Avhilst on tlie contrary, the sjACcies Avith a 
very large anterior and small posterior Aving, Avere considered liy me as 
belonging to the genus Pecten.”^- 
' Hall — Pal. Mew York, 1S84, V, Pt. 1, No. 1, p. xii. 
^ AA^aagen — Salt Range P’oss. {Pal. Itul.), 1881, 1, Pt. 2, p. 200. 
