MY AD OR A. 



Genus Mtadora, Gray. 

 Testa trigono-ovata, ineequivalvis, valvd sinistra plus mi- 

 nusve concavd, dextrd pland, rarb concaviusculd ; in- 

 equilateralis, latere postico rotundato, antico leviter 

 flexuoso, coarctato, in/erne plerumque truncato, de- 

 pression plano-coneavd sub umbones. Cardo : dea- 

 tibus in valvd dextrd duobus lateralibus, elongatis, ru- 

 dibus, ab umbone divergentibas, quorum postico piano, 

 subobso/eto ; in valvd sinistrd projeeturis sulcatis 

 duabus lateralibus denies recipientibus. Ligamentum 

 internum, in fared trigond centrali inter dentes inser- 

 tion, appendice testaced concavd seep) intend pro- 

 tectum. \~ulr<c iutiis uiarijaritaveic, jial/ii impressione 

 musculari antici sinuatd. 

 Shell triangularly ovate, inequivalve, left valve more or 

 less concave, right valve flat, rarely a little con- 

 cave ; inequilateral, posterior side rounded, anterior 

 slightly flexuous, contracted, generally truncated, 

 with a flat hollow depression under the umbones. 

 Hinge : two rude elongated lateral teeth in the right 

 valve diverging from the umbo, one on each side, the 

 posterior of which is flat and somewhat obsolete ; 

 in the left valve are two grooved lateral projections 

 receiving the teeth of the opposite valve. Liga- 

 ment internal, inserted in a triangular pit between 

 the teeth, often protected internally by a testaceous 

 appendage called the clavicle or hinge-piece. Valves 

 pearly within, muscular impression of the mantle 

 sinuated anteriorly. 

 The genus Myadora, introduced by Mr. Gray in his 

 account of the ' Shells of Molluscous Animals' in the 

 ' Synopsis of the Contents of the British Museum,' is 

 one that cannot fail to be appreciated ; nothing indeed 

 can more fully demonstrate the necessity for a new 

 generic allotment of certain species than the circum- 

 stance of their having been transported at different 

 times from one genus to another by the same author*. 



* " In an Appendix to a Catalogue of Shells collected in the 

 Australian and Polynesian groups of Islands liy Mr. S. Stutchbury," 

 says Sowerby, in his account of the genus Pandora, 'Species Con- 

 chyliorum,' Part I., " 1 have described under the name of I'mirim-n 

 brevis, a shell which I am now convinced is rather an Jnalina, in- 

 asmuch as its flat valve is destitute of the blunt tooth which cha- 

 racterizes the Pandora ; it differs also from them in having a sinus 

 in the muscular impression of the mantle, and in being possessed of 

 a small testaceous appendage attached to the ligament." This shell 

 however, it will be seen, hasnot the spoon-shaped processes of Anatina. 



The Myadora: partake of the characters of Anatina and 

 Pandora ■ and as they have been referred at times to 

 both of those genera, it is important to describe with 

 some minuteness the differences which entitle them to 

 generic distinction. 



In Anatina the hinge is composed of two hollow spoon- 

 shaped processes containing the ligament, protected in 

 some species by a moveable testaceous clavicle, which 

 crosses the dorsal axis of the shell on the posterior side * ; 

 as in Anatina truncata for example, a species now com- 

 monly obtained with this accessory hinge-piece complete. 



In Pandora, which is too flat and compressed a shell 

 to admit any structure like the spoon-shaped processes of 

 Anatina, the ligament is lodged in a cicatrix, protected 

 on the posterior side by a single central oblong tooth in 

 the right valve only ; the clavicle is dispensed with, but 

 the loss of it is in a degree supplied by a thickening and 

 folding over of the dorsal margin. 



In Myadora, which, being a thicker shell, requires a 

 hinge of more solid construction, the peculiarities above 

 noted in Anatina and Pandora, the clavicle of the former, 

 the folded margin of the latter, are united in the following 

 modified condition. The dorsal margin of the right 

 valve of Myadora becomes consolidated into a tooth-like 

 ledge or projection diverging from, on each side, the 

 umbo, fitting into grooved projections of similar con- 

 struction in the left valve ; and by the diverging of these 

 tooth-like projections, a compact triangular cavity is ob- 

 tained in the centre of each valve for the insertion of 

 the ligament, which in some of the species is walled in 

 as it were internally, not laterally as in Anatina, by a 

 moveable testaceous clavicle forming an angle with the 

 diverging ledges. 



The clavicles of Anatina and Myadora, it may be thus 

 observed, are very differently situated with respect to the 

 ligament, the one being a side appendage extending 

 across the dorsal axis of the shell, the other an internal 

 appendage forming an angle with the dorsal axis. 



Of the following ten species which I propose to refer 

 to this genus, the grand type, Myadora striata, is an in- 

 habitant of Port Nicholson, New Zealand; the remainder 

 are for the most part collected by Mr. fuming at the 

 Philippine Islands. 



* The side on which the proboscis or siphons are protruded is as- 

 sumed to be the anterior. 



