HEMIPECTEN. 



f genus, proposed l)y 

 the ' MoUusca of the 



Genus IIemipectf-n, Adam and Iti'i'w. 



Tesin ndharem, inaquivalvis, irregularis, InjaUna, valvd 

 siiperiori aiitice shiiplid, postice vix aurirtdatd, valvd 

 iiifiriiiri anlire xl,„pli,-i, pnslir:- rnii^pifur iitnivnluld, 

 infra am-in,l,ni, pnfnnii snauda ,'! dnilinilatd ; car- 

 diac cdiiidulu, Uijiiuicido levder iiiuryinidt, rartUagiiie 

 parvo ill cavitatem cmtraleni. 



Shell attached, iiiequivalve, irregular, hyaline, upiier \alve 

 simple anteriorly, very slightly auriculatcd posteriorly, 

 lower valve simple anteriorly, conspicuously auricu- 

 lated posteriorly, deeply sinuated and denticidated 

 beneath the auricle ; hinge edentulate, ligament 

 slightly marginal, with a small cartdage in a central 

 cavity. 



The subject of the present 

 Mr. Arthur Adams and myself 

 A''oyage of the Samarang,' is an incquivalve shell of the 

 lamellibranchiate family Pecliiiacea, intermediate in its 

 characters between Pecieii and Aiiomia. Like Anomia it 

 is a thin liyaUue substance, adhering to some foreign 

 body, and of an irregular growth, according to the inequa- 

 lities of its place of attachment. Like Pecteii the hiiine 

 consists of a slight marginal ligament, intersected in tlic 

 middle by a small triangidar cartilage, situated in the 

 hollow of a superficial cavity in each valve. The untk'r 

 valve is distinguished by a conspicuous auricle on the 

 posterior side, and beneath this is a sinus so deeply cut 

 in the direction of the hinge-margin as to remind one of 

 Ptdum, especially when presented with the under valve 

 closed upon the hollow of the upper one, as in Fig. 1 4, 

 3 b ; and the margin of this sinus, as indicated in sonic 

 of the Pecleiis, is fm-uished with a row of sharp erect den- 

 ticles. The shell bears some resemblance to Pedum, from 

 the circumstance of there being no corresponding sinus in 

 the upper valve ; but it is apparently only a character of 

 resemblance, not one of affinity. The shell has no uni- 

 bonal area ; nor are the sides of either valve reflected. 



The observations to be derived from the microscopic 

 structure of Hemipecien which has been kindly ex- 

 hibited to me by Dr. Carpenter, are, however, singularly 

 contradictory to the views presented by its external cha- 

 racters. Of the two .specimens collected, the texture and 

 composition of both valves consist of a hyaline semi- 

 pearlaceous lamina, presenting a series of closely-paeked 

 concentric lines, the interstices of which arc minutely rayed 

 with much finer lines. Submitted to the microscope, tlic 



ll'it v^ilvc in botii .■iiiecimeus (I'ig. 1 a, 3 (/,) is permeated 

 by copious tubuli, a character in which the genus agrees 

 with Pedniii and with some species of Lima, and (lifters 

 from Pecteri. Tliis tubularity Dr. Carpenter observed to 

 exist also in the upper valve of the colourless specimen 

 (Fig. 1 c), but not in the other (Fig. 2 c), so far as tlie 

 ]?ryozoon u])on its surface allows of an examination. 



Tiie upper valve of the coloured specimen Fig. 2 c. 

 possesses a rudimental sculptm-ing over its entire surface ; 

 but as it may have received this from the parasite, and 

 exhibits no other appreciable point of dift'erenoe, we have 

 not ventured to distinguished it specifically from (lie 

 white specimen. 



Species 1, (Fig. 1 and 3, Mus. lie 



IIeMII'ECTEN F0RBESI.\NUt 



Anomiffformi, tcu 



Hu 



nip. 



testa orbicular 



Ad 



hjalind, concentrice lineaid, 

 linearuni iiderstitiis eximii reticidatis ; valvd inferiori 

 plauulatd, auricula longitudinaliter radiatd, sinu pro- 

 fmidn, valvd snperiori convexd, vix aiiriculatd ; pel- 

 liiddu-alM, valvd snperiori interdum rufo-auraidio 

 radiatd. 



iEs' IIemipecten. Shell orbicidar, Aiiomia-shaped, 

 very thin, hyaline, concentrically liueated, interstices 

 between the lines delicately reticulated ; lower valve 

 flattened, auricle longitudinally rayed, sinus deep, 

 upper valve convex, scarcely auriculated ; transparent 

 white, upper valve sometimes rayed with orange-red. 

 us and Reeve, Pro. Zool. Soc. IStO, p. 133, Mo/- 

 liisca, pi. 1. f. 2. and Moll. Voy. Samarang. 

 /la/i. Sooloo Archipelago, Eastern Seas (dredged from a 

 coi-al and stony bottom at a depth of about foui-teen 

 fathoms) ; Belcher. 



Two specimens of this interesting new form were col- 

 lected during the voyage of H.M.S. Samarang, one smooth 

 and white, the other slightly sculptured in a decussately 

 corrugated style, probably from the effect of the Bryozoon 

 which covers it, and rayed with orange-red. The under 

 valve is smooth in both specimens, sliowiiig it to have 

 been attached ; the upper valve is more or less covered in 

 both with various parasitic objects. 



We have the pleasure to name the species in honour of 

 Professor Edward Forbes, who notices the genus, in ids 

 valuable work on the British Mollusca, as aflPording a 

 curious intermediate link between Pectcii and Anomia. 



September, 1.S4'.) 



