REPORT ON THE LAMELLIBRANCHIATA. 69 



convex, depressed or a little concave l>elund,a faint angulation extending from the umbones 

 to the lower posterior end, thin, dirty whitish, finely concentrically striated, marked with 

 more distinct lines of growth here and there, and everywhere very minutely roughened 

 or subgranulated. The dorsal line is very oblique and moderately arched anteriorly, less 

 descending and rectilinear behind, forming with the side margin, which is almost straight, 

 and slopes inward or towards the middle of the slightly curved lower margin, a rather 

 sharp angle. The hinge is not quite like that of TJirncla fruncata of Brown, the process 

 to which the ligament attaches itself lieing rather slighter, distinct from, and soldered to 

 the hinge-line. The muscular scars and the sinus in the pallial line are similar. 



Length 23 mm., height 19, diameter 10. 



Length 23 mm., height 18, diameter 9|. 



Habitat. — Royal Sound, Kerguelen Island, in 20 to 60 fathoms ; Betsy Cove, in 

 20 to 25 fathoms ; Marion Island, in 50 to 75 fathoms ; and Prince Edward Island, in 

 100 to 150 fathoms. 



This is the southern representative of the Greenlandic species Thvacia truncata, and 

 indeed differs so slightly from it that it is with considerable hesitation I venture to 

 describe it as distinct, being mainly influenced to do so by the difference of locality. It 

 is not so equilateral, has the ventral margin straighter, the posterior end more abrujitly 

 truncated, the extremity being at the upper angle, whilst in the boreal form the low^er end 

 is usually the most prominent. The hinder dorsal slope in the present species is a little 

 reflexed or bent over in botli valves, forming an indistinct area, which in Thracia truncata 

 is scarely traceable. A more tangible difterenee is in the slighter shelly process which 

 supports the ligament, being, as it were, a separate ])iece joined on to the hinge-line. 



Tliracia iratsoni, n. sp. (PI. VI. figs. 5—56). 



Testa magna, eomplanata, fere, aequilateralis, utrinque dorsaliterque aliquanto hians, 

 all)ida, concentrice fortiter plicato-rugata, scabra, granulata, transversim oblonga, antice 

 rotundata, postice obbque truncata, margine ventrali rectiusculo rotundatim in latum 

 anterius ambiente. Margo dorsi auticus paulo obliquus, vix arcuatus, posticus valvse 

 dextrse mediocriter concavse horizontalis, in valva sinistra siibplana leviter obliquus, iufi-a 

 umbonem concaviusculus. Umbones parvi, acutissimi, postice inclinati. Area excavata, 

 subacute marginata. Ligamentum externum utroque ajMcis latere pi'olongatum, antice in 

 fossula parva marginam intra situm. Cartilago interna magna, triangularis, in receptaculo 

 calloso crasso inclusa. Cicatrix muscularis posterior fere circularis, anterior elongata, 

 irregularis. Pallii sinus profundus, latus, ad apicem rotundatus. 



This magnificent large species is much compressed, very elongate, not quite twice as 

 long as high, almost equilateral, rounded in front, oljliquely truncated at the opposite 



