MEGATHYRIS. 359 



shell intended he was acquainted with), which represents 

 the T. vitrea, a closely allied species with an entire fora- 

 men, and a shorter cardinal appendage. From the state- 

 ment in the Dictionary that the single specimen dredged 

 alive in Dublin Bay had been deposited in the Museum of 

 the Dublin Society, a search has more than once been insti- 

 tuted for it, the result of which shows that a Terebratula 

 thus marked is certainly present (Thompson), but is the 

 psittacea, a species that by no means harmonizes with the 

 description of the shell in question, and which, moreover, 

 was well known to, and adequately described by the Doctor. 

 Either, then, the locality has possibly been appended to 

 the wrong diagnosis (if, indeed, there be any connection 

 between the ticketed example and Tin-ton's observation), 

 or not improbably the individual described, being, in truth, 

 the exotic vitrea, having been detected as spuriously in- 

 digenous, was purposely left out in the later and more 

 original " Dithyra Britannica." 



MEGATHYRIS. A. d'Orbigny. 



Shell inequivalve, inequilateral, semi-orbicular, com- 

 pressed, often strong, surface smooth, or with radiating 

 ribs, always punctated. Area large, triangular, with a 

 large incomplete foramen under the beak of the upper 

 valve, and impinging on the lower; no deltidium. No 

 free apophysary system, but one or more ribs or dissepi- 

 ments rising from the inner surface of the smaller valve. 



Animal with contorted or spiral arms fixed to the 

 margin of the apophysary ribs and cardinal teeth. 



The very rare and minute Terebratula cistellula of Mr. 

 Searles Wood appears to belong to the genus Megathyris 

 of D'Orbigny, of which Terebratula detruncata, and some 



