50 SIXTEENTH KEPORT ON THE CABINET OF NAT. HISTORY. 



seems necessary to repeat some of the characters of the genus in 

 this connexion. 



GENUS MERISTELLA (Hall, 1860). 



The genus includes terebratuloid or athyroid forms which are ovoid, 

 more or less elongate, sometimes elliptical in outline, and not 

 unfrequently transverse or subcircular. Yalves unequally con- 

 vex, with or without a median fold and sinus; and this feature, 

 when present, usually confined to the lower half of the shell. 

 Ventral beak more or less closely incurved ( when closely in- 

 curved, apparently imperforate), terminated by an aperture, 

 the lower side of which may be formed by the umbo of the 

 dorsal valve or by a deltidium : area none.* Yalves articulating 

 by teeth and sockets. 



Surface smooth or marked by fine concentric lines of growth (not 

 lamellose), and indistinct or obsolescent radiating strise, which 

 are usually more conspicuous in the cast or exfoliated surfaces 

 than on the exterior. Shell fibrous. 



The ventral valve is much thickened on each side towards the 

 beak, and the rostral cavity margined by flattened dental lamellge, 

 which extend downwards to the commencement of the muscular 

 impression, and terminate at the edge of the shell in blunt tooth- 

 like processes. The muscular impression forms a somewhat broadly 

 triangular depression in the valve just below the rostral cavity. 

 In the cast of this valve, we have the reverse of these features. 



In the dorsal valve there is a strong hinge-plate or process, the 

 prominent part of which is broadly triangular, somewhat depressed 

 or spoonshaped in the centre, and supported below by a median 

 septum which reaches from one-third to one-half the length of the 

 valve, and on each side marked by deep dental fossets, while the 

 anterior angles are produced into the crura which support the 

 internal spires. 



Spires arranged as in Athyris and Merista, being a double 

 cone with the apices directed outw^ards. From the lower lateral 

 margins of the cardinal process or hinge-plate, there is a callosity 

 extending beneath and anterior to the dental fossets, and joining 



• Those species with the ventral valve closely incurved are apparently imperforate, 

 Bince no foramen is visible above the umbo of the dorsal valve. In the separated valves 

 of these species, I have not seen any deltidium : an open triangular space exists above 

 the points of the dental lamellae, and this communicates with the open cavity of the 

 valve, 



