﻿ARCKLEDISCUS. 



143 



Characters. — Shell lenticular, rounded, bi-convex, somewhat unsymmetrical. 

 Periphery determined by the last revolution of the tubular cavity. Margin angular or 

 slightly rounded ; entire except near the orifice where the shell, owing to diminished 

 thickness, is often broken and irregular. Aperture, the open unconstricted end of the 

 tubular cavity, rounded or crescentic, simple. Diameter, inch (1*0 mm.). 



The brief notice above referred to, read before the British Association at its Bradford 

 Meeting, and subsequently published in the Annals of Natural History {loc. cit.) contains an 

 account of the structure and affinities of this interesting type as far as at present known — 

 continued observations having resulted in little beyond the confirmation of the views 

 therein expressed and the extension of our knowledge of its distribution. 



Externally the specimens of Archcediscus Karreri are lenticular discs, seldom more 

 than a twenty-fifth of an inch (TO mm.) in diameter, and a fiftieth of an inch (0"5 

 mm.) in thickness and never quite symmetrical. They often present an appearance as of 

 laminated structure, and altogether bear a superficial resemblance to minute 

 Nummulites. 



The interior will be best understood by comparing it to a tube coiled upon itself in 

 constantly varying directions, the periphery being determined by the last circlet of the 

 coil. The tube, which represents the cavity occupied during life by the main body of 

 the animal, is never subdivided into chambers either by constriction or by true septa, 

 and it increases in size with each successive turn. The earlier portion in one 

 section which has been measured has a transverse diameter of about -g^y of an inch 

 (0 04 mm.), the later portion of an inch (013 mm.), though in most cases the 

 difference is scarcely so great as these figures imply. The shape of the tube also varies 

 considerably. Its transverse section at times represents about three-quarters of a circle, 

 the truncate or flattened side facing inwards ; at others it exhibits an irregularly 

 crescentic or saddle-shaped contour, the concave surface of which embraces portions of 

 the preceding convolutions to a greater or less extent. 



The coil terminates externally on the periphery of the disc, and the majority of 

 specimens have an appearance as though a part of the end of the tube had been broken 

 away (as sometimes observable in Nummulina), owing probably to the greater delicacy 

 and tenuity of the newly deposited shell-substance. The mouth of the tube, which forms 

 the general aperture of the shell, appears not to have been constricted or otherwise 

 closed in. 



A shell formed in the manner described would present an irregular surface, were the 

 walls of the tube of equal thickness throughout, but in reality the exterior is even and 

 smooth. A transverse section of the entire fossil (PI. XI, fig. 4) shows that this is due 

 to a somewhat remarkable thickening of the shell-wall, especially on its lateral surfaces, 

 most observable near the centre of the disc, and usually to a greater extent on one side 



