PECTUNCULUS. 



Plate I. 



Genus Pectunculus, Lamarck. 



Testa orbicularis, sublenticu/aris, tsquivalvis, sub/cqui late- 

 ralis, epidermide pilosd sicpi indutd ; umbonibus pa- 

 rum separatis, area intermedia pared ; valvarum 

 marginibus plerumque crenulatis. Cardo arcuatus, 

 dentibus numerosis obliquis, mediants sespissime ob- 

 solelis. Ligamentum externum. 



Shell orbicular, somewhat lenticular, equivalve, rather 

 equilateral, often covered with a hairy epidermis ; 

 umbones but little separated from each other, inter- 

 vening area small ; margins of the valves generally 

 crenulated. Hinge curved, teeth numerous and ob- 

 lique, middle ones mostly obsolete. Ligament ex- 

 ternal. 



The word Pectunculus, which Klein, Lister, and many 

 writers of the sixteenth century applied indiscriminately 

 to different kinds of bivalve shells, was selected by La- 

 marck for the purpose of distinguishing such of the 

 Linnaean Area as have their teeth set in a semicircle or 

 curve. They differ, however, from the modern Arete in 

 other not less important particulars, and a good generic 

 distinction was discovered by Poli on examination of the 

 soft parts. Lamarck mentions twenty species of recent 

 Pectunculi, and some few have been since described by 

 Sowerby and Broderip. The total number of species 

 now before me is about fifty ; many of them are new, 

 and by far the greater portion have not hitherto been 

 figured. 



Species 1. (Mus. Cuming.) 



Pectunculus intermedius. Pect. tesld suborbiculari, 

 subglabrd, subdepressd ; ulbidd, castaneo umbones 

 versus pallide undato-radiatii ; striis radiantibus, 

 subdistantibus, decussatis ; intus alba, marginibus 

 crenatis ; epidermide crassd, pilosd. 



The intermediate Pectunculus. Shell rather orbicu- 

 lar, smooth, and a little depressed ; white, radiated 

 towards the umbones with pale chestnut zigzag 

 lines ; striae or lines of growth radiating, rather 

 distant, crossing each other transversely and longi- 

 tudinally ; interior white, margins crenated ; epi- 

 dermis thick and hairy. 



Broderip, Proe. Zool. Sue, I s :; -J , p. 126. 



Hab. Iquiqui, coast of Peru. (Found in coarse sand at 

 the depth of ten fathoms.) Cuming. 



In all the specimens I have seen of this shell the epi- 

 dermis is removed from about the umbones in the pecu- 

 liar manner represented in the plate, and (as Mr. Bro- 

 derip notices) the pale chestnut radiating zigzag lines 

 which adorn that part are often effaced to a certain ex- 

 tent by erosion or decomposition. 



Species 2. (Mus. Cuming.) 



Pectunculus ovatus. Pect. testd obovatd, convexd, gla- 

 bra, lineis transversis minutissimis ornatd ; albente, 

 umbonibus castaneo pullidi notatis : intus alba, mar- 

 ginibus crenatis; epidermide teuui, subvillosd. 



The oval Pi:( rrxcuLus. Shell exactly oval, convex, 

 smooth, ornamented with very minute transverse 

 lines ; whitish, marked with pale chestnut towards 

 the umbones ; interior white, margins crenated ; 

 epidermis slight and downy. 



Broderip, Proc. Zool. Soc, 1832, p. 126. 



Hab. Island of Lobos, Pacific Ocean. (Found in coarse 

 sand at the depth of seventeen fathoms.) Cuming. 



This shell, which has a much lighter and more downy 

 epidermis than the preceding, may be easily recognized 

 by its peculiarly oval form. 



Species 3. (Fig. a and b, Mus. Cuming.) 



Pectunculus giganteus. Pect. testd valde. convexd (ju- 

 niore depressd, fig. b.), solidd. giganted, longitudina- 

 liter stria/d, striis con/ignis, regu/aribus, et su/eatd, 

 sulcis fere obsoletis ; albd, inferne castaneo timid. 

 superni maculis rubido-fuseis numerosis undatis, con- 

 tingentibus, profuse et vivide pictd ; intus alba, mar- 

 ginibus (in adultd) castaneo-nilidis, crenatis; epi- 

 dermide crassd, pilosd. 



The gigantic Pectunculus. Shell very convex (de- 

 pressed in an early stage of growth, see fig. b.), 

 solid, and of very large size, longitudinally stri- 

 ated ; striae close and regular, grooved in the same 

 direction, grooves almost obsolete; white, stained 

 towards the lower pari with chestnut; middle and 

 umbonal portion of the shell profusely and vividly 

 painted with waved reddish-brown spots, touching 

 each other for the most part on both sides; inte- 

 rior white, margins (in the adult shell) of bright 



Feb. 1843. 



