109 



CONCH ACE A. 



CONCHACEA. 



110 



of colour), and others as founded merely on the difference of age, 

 T, Muroltuca, for example. 

 Lamarck divided the species into 



1. Those with the shell transversely oblong. Example, T. 

 radiata. 



2. Those with the shell orbicular, or rounded oval. Example, 

 T. Kobinata. 



De Blainville divides the genus thus : 



1. Subtriquetral species. Example, T. bimacitlata, 



2. Elongated species, but which have the posterior side shorter 

 and narrower (plus e"troit) than the anterior. Example, T. radiata. 



3. Oval, or suborbicular, and nearly equilateral Example, T. 

 tcobinata. 



4. Equilateral species, sufficiently elongated, almost without a 

 fiexuoua fold ; two divergent cardinal teeth, and two distant lateral 

 ones, of which the anterior is but little distant from the umbo. 

 (TetUnidet, Lam.) 



1. TeHina roilrata. 2. Trllinidrl. 



The fossil species are recorded as occurring in the Supracretaceous 

 group, in the Cretaceous group, and in the Oolitic group (Coralline 

 Oolite, Yorkshire ; Kimmeridge Clay ; Bernese Jura). Sir R. Murchison 

 mentions two species (probably) in the Salopian outlier of Lias. 



A mpkidetna. Shell suboval or rounded, of little thickness, longer 

 than it is high, inequilateral, sometimes a little gaping ; hinge with 

 one or two cardinal teeth, and sometimes lateral teeth more or less 

 projecting ; ligament double ; one ligament external and short, the 

 other internal and fixed in a narrow (e'troite) hollow of the hinge. 



As the genus was left by Lamarck, it would appear to be widely 

 spread, for it is recorded as occurring in the European seas (Northern, 

 English Channel, Mediterranean) ; those of Australia and the south ; 

 and on the coasts of Brazil. But it should be remembered that A. 

 rorbuloida, Lam., Mya Norwtgica, Chemn., is the example given by 

 Deshayes for his genus Oiteodetma, while A . glabretia (seas of Australia 

 and Kangaroo isles) is one of his Mwjdexmaia. The species, which 

 are tolerably numerous in their undisturbed state (Amphidftma, 

 Lam.), are said to have been found in sands and mud at depths 

 varying from the surface of the sea to forty fathoms. Lamarck gives 

 sixteen species ; Mr. G. B. Sowerby has added twelve, brought home 

 by Mr. Cuming. (' ZooL Proc.') 



A . rarieyatum may be taken as an example of the genus. It is found 

 on the coast of Brazil. 



Amphidenma tariegatum. 



But few species have been found fossil. 



Maodetma (Deshayes). Animal inclining to oval or subtrigonal, 



flattened; lolies of the nniitl" united for two-thirds of the posterior 



ii, and provided, at their posterior extremity, with two short 



i|j|i"ii* |>rnloiii;-d within by a very delicate membrane; foot very 



much flattened, quadrangular, hidden in part by the branchia>, which 



are short, truncated, and fixed (soude'es) posteriorly, the external pair 

 smallest and subarticulated. Shell oval, transverse or triangular, 

 thick and ordinarily closed. Hinge with a spoon-shaped hollow, 

 straight and mesial for the ligament, and, on each side, an oblong and 

 simple tooth. (Deshayes.) 



M. Deshayes remarks that the shells of this genus are easily 

 recognised. The shell is always thicker than that of the Mactrce : 

 they are more compressed, more completely closed (raieux fermtSes) 

 and hi this respect approach the Cra&satcllce. The hinge is particularly 

 remarkable ; in the middle of the border and immediately below the 

 umbo is placed a spoon-shaped triangular deep hollow, the border 

 of which projects within the valves as in the greater part of the 

 Lutrarice. On each side of this spoon-like process, in which the 

 ligament is inserted, is seen in each valve a large thick tooth, and 

 behind is a hollow to receive the tooth of the opposite valve. Muscular 

 impressions unequal ; the anterior largest, elongated ; the posterior 

 somewhat rounded. The pallial impression in the species which 

 approach the Mactra has a moderate posterior sinuosity which 

 diminishes more and more in proportion as the species have more 

 resemblance to the Crassatdtce. The sinuosity exists however in all 

 the species of the genus. 



Cumingia (G. B. Sowerby). A genus which should be placed near 

 to Amphidesma. It is remarkable for the dissimilarity of the hinge 

 of the two valves, one having a strong lateral tooth on each side of 

 the ligament, and the other being entirely destitute of lateral teeth. 

 Having only met with a small West Indian species, we could not 

 venture to consider this genus as established, until Mr. Cuming showed 

 us several species in his rich collection of South American and Pacific 

 shells, one of which is sufficiently large to show the characters 

 distinctly. ('Genera of Recent and Fossil Shells,' No. 40.) Mr. Sowerby 

 characterises the shell as inequilateral, equivalve, with the anterior 

 side rounded and the posterior rather acuminated. A single small 

 anterior cardinal tooth observable in each valve : one strong lateral 

 tooth on each side of the hinge in one valve, but no lateral tooth in 

 the other valve ; ligament internal, and affixed to a somewhat spoon- 

 shaped pit in each valve. Muscular impressions two in each valve, 

 lateral and distant, the anterior irregular and oblong, the posterior 

 rounded. A very large sinus in the muscular impression of the mantle. 



The species are found in the tropical seas as far as is yet known, in 

 clay, mud, and sand, in the fissures of rocks, at a depth varying from 

 the surface of the sea to six fathoms. No fossil species known. 

 Example, Cnmingia mutica (Sow.). 



Cumingia mutica. 



Mactra. Animal oval, somewhat thick, with the borders of the 

 mantle thick and simple, furnished posteriorly with two tubes but 

 little elongated and united ; branchial lamina; small and nearly equal ; 

 foot oval, trenchant, very long, angular. Shell transverse, inequi- 

 lateral, subtrigonal, sometimes a little gaping at the sides ; umbones 

 protuberant ; hinge with one cardinal tooth, folded into the shape of 

 the letter V, the point being nearest the umbo and the branches 

 diverging from it ; posterior to this and very close to it is a very 

 thin sharp tooth ; sometimes the branches of the folding tooth are 

 separated at the base, forming two diverging teeth ; ligamental pit 

 immediately behind the angular tooth and projecting within the 

 shell. Lateral teeth, two on each side in one valve, one on each side 

 in the other, diverging from the umbones, and very near the margin, 

 thin, mostly elongated, and the inner ones more prominent than the 

 outer, but in some species very short, in the thicker species perpen- 

 dicularly striated. Muscular impressions two, lateral, distant ; pallial 

 impression with a small sinus. Ligament consisting of two portions 

 (as usual), one, by far the larger, internal ; the other external. In 

 some species the umbones are separated, and the ligament forms a 

 deep pit extending both within and without to the point of the beaks : 

 of this M. Spenfjieri is an example. 



" This genus," says Mr. G. B. Sowerby, " contains a great number 

 of species, some of which are handsome and others very singular 

 shells ; upon examining a number of species we think it might be 

 desirable to divide it into several genera, because we find several 

 distinct forms in it." It is found in Europe, East and West Indies, 

 Africa, North America, &c., buried generally in sandy mud and sands 

 at a depth varying from the surface of the sea to 12 fathoms. 



The species are numerous. Deshayes, in his Tables, gives thirty-two 

 living ; in his edition of Lamarck thirty-three ; but in his opinion 

 one of these, M. donacia, is not a Mactra but a Meeodesma, and 

 others are repetitions or varieties. 



