238 BULLETIN OF THE 
Gexnus PECTUNCULUS Lamarck. 
Pectunculus undatus Linn. 
Arca undata Linné, S. N., ed. xii., p. 1142; Hanley, Shells of Lin., p. 97. 
Pectunculus undulatus Lam., An. s. Vert., VI. p. 50, 1819. 
Pectunculus lineatus Reeve, Conch. Icon., pl. v. fig. 25, 1843. 
Pectunculus scriptus (Born) Reeve (young shells). 
Pectunculus hirtus Phil. Zeitschr. fiir Mal., 1846, p. 191. 
2 Pectunculus angulatus Lam. 
2 Pectunculus pennaceus Lam. (= decussatus Linn.). 
Pectunculus passus Conrad, Tert. Foss. U.S., p. 64, pl. xxxv. fig. 3, 1844; Tuomey & 
Holmes, Plioe. Fos. S. C., p. 48, pl. xvii. fig. 3 (good), 1855. 
Pectunculus quinquerugatus Conrad, Ann. Journ. Sci., IV. p. 346; T. & H., 1. c., p. 49, 
pl. xvii. fig. 4, 1855. 
Pectunculus carolinensis Holmes, Post Pliocene Fos. 8. C., p. 15, pl. iii. fig. 4, 1858. 
(Not of Conrad “ 1838 ” = 1844.) : 
Pectunculus tricenartus Conrad, Tert. Fos. U. S., p. 63, pl. xxxv. fig. 1, 1844. 
Pectunculus parilis Conrad, 1. c., p. 64, pl. xxxvi. fig. 2, 1844. 
A single valve of the scriptus variety at Station 127, Santa Cruz Island, in 
38 fms. 
I have been unable to examine any specimen of P. pennaceus Lamarck (= P. 
decussatus L. sp.), or at least none of the specimens so named which I have 
been able to examine have had at one end of the beaks the heart-shaped area 
described by Lamarck and Hanley. Iam therefore unable to say whether it 
and its synonyms should find a place here, as suggested by D’Orbigny and 
Krebs, both good judges. For the rest, it is evident that an absurd number 
of species have been made of this group, especially in fossils, where a man is 
allowed to describe a species from one valve without adverse comment. 
A careful examination of a large number of good specimens of this species of 
Peetunculus will show any competent observer, — Ist, that the reticulated sculp- 
ture is always present on the umbones of a perfect shell, and its greater or less 
extension and uniformity over the valves in the adult varies with the individ- 
ual; 2d, that the hinge in this group is very mutable within certain limits, and 
undergoes great changes with age, and the number of teeth is greater in the 
adult than in the young; 3d, that a certain lateral expansion and angulation, 
which are very marked in some specimens, are variable characters; 4th, that, 
as one goes south, the shells of this species become more brightly colored, more 
inflated, more purely porcellanous, and show a tendency to equalizing the 
strength of the radiating and concentric sculpture, thereby producing reticu- 
lation, which governs the disposition of the pilose epidermis and alters the 
aspect accordingly; they are also rather smaller when full grown. I have 
come to this opinion through the study of a large series collected by the U.S. 
Fish Commission, and another existing in the Jeffreys collection, covering 
