Thracia.} PECEC'YPODA. 1023 



Genus 1. THRACIA, Blainville, 1824. 



Thracia (Leach MS.), Blainville, Diet. Sci. Nat., xxxii, 1824, 347. Rupicola, 

 Fleuriau, 1802; not of Brisson, 1760. Anutina, Blainville, 1825; trans- 

 ferred to Osteodesma. Hemicyclonosta, Blainville, 1827 ; not of Deshayes, 

 1835. Odoncinetus, 0. G. Costa, 1829. Ixartia, Leach, 1852. Corymya, 

 Agassiz, 1843. Cyathodonta, Conrad, 1849. Rupicilla, Schaut'uss, 1867. 

 Homceodesmu, Fischer, 1887. Pelopia, H. Adams, 181)8. Type : T. cor- 

 hitloidea, Blainville. 



Shell concentrically striated, with more or less fine superficial 

 granulation and very delicate periostracum ; subrostrate, slightly 

 gaping behind ; slightly inequivalve, the right valve larger ; the beaks 

 in contact and usually perforated by friction on each other, the hinge- 

 plate fissured below them and edentulous ; the ligament external, 

 the resilium more or less sunken and with, in most cases, a short 

 transverse lithodesma in front of it, occupying the fissure in the hinge- 

 plate ; pallial line with a moderate sinus, margins of the valves entire ; 

 the nymphs in the typical forms do not project greatly from the hinge- 

 margin ventrally, and are more or less elongated ; the shell is destitute 

 of nacre. 



Distribution. Seas of Europe, east coast of North America, Chinese 

 Sea, Australasia, &c. 



Fossil in the Secondary and Tertiary. 



Vernacular Name. Lantern-shell . 



KEY TO SPECIES. 



A. Shell equivalve, the anterior end longer . . . . . . vifrea. 



B. Shell inequivalve, the left valve much flatter than the right ; equi- 



lateral or subequilateral . . . . . . . . trnnsenna. 



1. Thracia transenna, n. sp. Plate 55, figs. 9. a. 



Shell transversely elongate, thin and fragile, equilateral, inequi- 

 valve, the right valve moderately convex, the left flattish, broadly 

 truncated behind, finely concentrically striated. Beaks not much 

 raised, small, acute, incurved. Anterior end narrowly convex, the 

 dorsal margin straight, sloping ; posterior end vertically broadly trun- 

 cated, the dorsal margin lightly concave, angled at the truncation ; 

 basal margin broadly rounded. Sculpture consisting of numerous pro- 

 minent concentric strise, stronger on the right valve, which has 2 pos- 

 terior oblique ridges, both broadly rounded, the lower descending 

 from the umbo to the lower end of the truncation, the upper one to 

 the middle of it ; the microscopic sculpture consists of very fine denso 

 concentric lines, cut up by somewhat irregular and rather distant 

 radial grooves. Colour yellowish-white. Interior white, dull, some- 

 times slightly pearly outside the pallial line. Margins smooth. Hinge 

 with a small lithodesma. Adductor-scars slightly unequal, the anterior 

 more elongated. Pallial sinus moderately deep and broad, obliquely 

 truncated, the lower end extending as far as the vertical from the 

 beak. 



