M A C T R A . 



Plate I. 



Genus MACTRA, Linnaus. 



Testa ovato-trigona vel trmisversa, laieribus pauUsper hians; 

 umbonibus prominentibus. Dens cardiiialis hi iitrdque 

 valvd compresms, pUcato-caiialicidatus, cum adjectdfo- 

 veold intus promimdd. Denies laterales duo conipressi, 

 uirhiqiie props cardinem inserti ; Hgamentum internum, 

 in foveold cardinaU insertimi. 

 Shell ovately triangular or transverse, a little gaping at 

 the sides ; umboes prominent. Cardinal teeth in 

 each valve compressed, plicately channelled, with the 

 interlying pit rather prominent within ; two lateral 

 compressed teeth inserted on each side near the 

 hinge ; ligament internal, inserted in the hinge pit. 

 There is no genus of bivalves which it is more easy to 

 determine than Mactra. The shell has, with little varia- 

 tion, a ciiaracteristic triangular form, more or less trans- 

 versely ovate or oblong; and the hinge is distinguished by 

 a prominent central triangular tooth in one valve, which 

 closely interlocks in a corresponding pit in the opposite 

 valve. The Mactrm are mostly void of colour, but some 

 are very prettily tinged and rayed with rose or pur- 

 ple. In sculpture they are also deficient, compared with 

 the Tellinidm or Venerida, having seldom more than 

 concentric grooves and strife or plait-like ridges. The 

 anterior or front half of the shell is, with few exceptions, 

 tiie shorter, and this is sometimes concentrically grooved 

 and striated while the posterior side is smooth. Beneath 

 the umboes on the anterior side a neat heart-shaped lunule 

 is generally formed, and the striae of the valves mostly 

 converge on the shield of this in a more regular and ele- 

 gantly developed state. On the posterior side the Uga- 

 mentary area is more broadly spread, and mostly angled 

 on each side. Some of the Mactra, more especially those 

 inhabiting the Northern Seas, are ponderous and Mya- 

 like; the majority of the species inhabiting the tropical 

 seas are thin, some almost transparent, and most elegantly 

 wave-wrinkled and striated. The localities noted in the 

 following monograph will show that the genus is pretty 

 evenly distributed over the globe, from the Arctic Seas 

 along both sides of tlie African continent to Van Diemen's 

 Land in the eastern hemisphere, and from the North Ame- 

 rican Seas to Panama and Columbia in the western. 



Lamarck enumerated about thirty species in his Anira. 

 sans vert., but several have been described subsequently 

 by difl'ereut authors, and the genus has lately undergone 



a complete revision by JL Deshayes, who has been staying 

 in this country for the purpose, while assisting in the ar- 

 rangement of Mr. Cuming's and our national collections. 

 The species are brought up in the present monograph to 

 a hundred and twenty-five in number. It will be seen that 

 several well-marked groups are included in this number, 

 all belonging to the Lamarckian genus Mactra, but deno- 

 minated as genera by the more advanced conchologists, 

 under the names of Mulinia, Scliizodesma, Cypricia, Raeta, 

 Standella, Spisnta, etc., as will be seen by the synonyms. 



Species 1. (Mus. Brit.) 



Mactra triangularis. Mact. testd tr'ujono-elongatd, 

 compressd, solidd, iiKeriuilaterali, latere antico paulo 

 breviore, ambobus compresso-acuininatis, transversim 

 plicato-liratd, fiiscescente-albd, maculis fuscescentibua 

 parvis sparsim aspei'sd ; umbonibus parvis, angulari- 

 bus, approximatis ; lunuld aredque peramplis, lanceo- 

 lato-ovatis, medio elevatis, fusco-strigatis. 



The triangular Mactra. Shell trangularly elongated, 

 compressed, solid, inequilateral, anterior side a little 

 the shorter, both sides compressly acuminated, trans- 

 versely plicately ridged, brownish-white, sparingly 

 sprinkled with small brown spots ; umboes small, 

 angular, approximated ; lunule and area very large, 

 lanceolately ovate, raised in the middle, streaked with 

 brown. 



Lamarck, Anim. sans vert. (Deshayes' edit.) vol. vi. 

 p. 103. Enc. Me'th. pi. 253. f, 3 a, b, c. 

 Spisula triangularis, Gray. 



Sab. ? 



An extremely rare species, of a peculiarly triangularly 



elongated wedge-shape, sprinkled with small red-brown 



spots. 



Species 2. (Mus. Cuming.) 



Mactra Venerifoemis. 3facf. testd orbiculato-subtri- 

 gond, inflatd, convexissimd, cordiformi, subaquilaterali, 

 antice obtusd, postice longiore, angulald, transversim 

 regulariter striatd, ad umbones leevigaid, squalide albd 

 vel grised, postice intus extusque violaceo-tinctd ; um- 

 bonibus tumidis, pi'ominentibus, oblique cordatis ,■ lu- 

 nuld aredque latis, ovatis, regulariter plicatis ; sinu 

 pallii lato, subsemicirculari. 



April, 1854. 



