no INTRODUCTION. 



and deltidium in the attached valve; no calcified supports for the arms, which were 

 evidently fleshy and spirally coiled. 



Obs. The place this genus should occupy in the classification, does not seem to have 

 been quite satisfactorily established, I have therefore followed Professor King in constituting 

 it for a small sub-family, whence it may be easily removed, should future discoveries 

 require it to be admitted into any of those already established. 



Genus — Davidsonia, Bouchard, 1849. 



Type—D. Verneuili, Boueh. Int., PL VIII, figs. 186—193. 



Thecidea (prisca), Goldfuss, MS. Mus. of Bonn. 



Lept^na? Be Verneuil, 1845. 



Davidsonia, Bouchard, Be KonincJc, King, Schnurr, &c. 



Shell transversely oval, with thick unequal valves, fixed to marine bodies by a por- 

 tion of the surface of the ventral valve, filling outward irregularities, but not reproducing 

 the same internally ; the unattached portion of the dental valve rises abruptly, especially 

 in front ; area more or less defined, and divided by a convex triangular pseudo-deltidium, 

 considerably notched at its inner margin ; upper or socket valve thick, slightly convex or 

 concave ; external surface smooth and marked by concentric lines of growth which extend 

 uninterruptedly over the rudimentary area. Valves articulating by means of strong 

 teeth placed on either side at the base of the fissure close to the hinge line, and corres- 

 ponding with sockets in the upper valve. In the interior of the attached valve a hollow is 

 visible under the pseudo-deltidium, aff'ording space for the cardinal muscular process of 

 the other valve ; between and below the dental projections are situated the muscular 

 scars, which extend to about a third of the length of the shell ; these consist of two deep 

 oval impressions, probably left by the cardinal muscles, and divided by a broad flattened 

 and sinuated mesial elevation, to which the adductor was possibly affixed ; the greater 

 portion of the interior is occupied by two conical elevations projecting more or less be- 

 yond the level of the valve ; the lateral and frontal portions of these solid cones exhibit 

 five or six semicircular or spiral projecting ridges, diminishing in surface and width as they 

 approach the summit of the cone, but not distinctly extending over the portion facing the 

 cardinal edge, which is covered to its extremity with the same minute indentures or punctures 

 which are seen on those portions of the internal surface unoccupied by muscular scars ; the 

 cones are separated by a longitudinal furrow, or space, which blends with the raised border 

 surrounding the interior of the valve. In the interior of the upper or dorsal valve, between 

 the largely developed socket walls, and close to the hinge line, is situated a small cardinal 

 process, under this the deep adductor impressions are seen lying in a semicircular 



