LOWER HELDERBERG ROCKS. 219 



species ; and I find that the fossil described by me as Atrypa concinna 

 in the Report of the Fourth Geological District (1843), is another species 

 belonging to the same group of fossils, in which both the external cha- 

 racters and internal structure differ ' so essentially from any of the de- 

 scribed genera of Brachiopoda as to constitute a distinct genus ; and 

 which, from the general nucleolar character of the known species, I 

 propose to designate Nucleospira. 



Gmus ]¥rCLEOSPraA (n.g.). 



[ Gr. irupijv, nucleus ; d-Ksipa, spira.] 



Shell spheroidal or transversely elliptical, more or less gibbous or ventri- 

 cose, furnished with internal spires as in Spirifer : hinge line shorter 

 than the width of the shell; cardinal extremities rounded : valves 

 subequal, articulating by teeth and sockets. Ventral valve having the 

 beak extended beyond the opposite valve, and beneath it a triangular 

 depression or area, which sometimes terminates in a shallow spoon- 

 shaped pit ; on each side of which, at the base, is a strong tooth : a 

 narrow ridge or septum extends along the centre of the inner side of 

 the valve, from beak to base. Dorsal valve furnished with a strong 

 spatulate cardinal process, which, rising vertically from the cardinal 

 margin, is closely grasped at its base by the cardinal teeth of the other 

 valve ; and thence bending abruptly upwards, and expanding, is pro- 

 jected into the cavity of the opposite beak, lying close upon the under 

 side of the false area. This process is grooved or depressed in the centre 

 of the upper side, so as to leave between it and the arch of the ventral 

 beak a narrow space for the passage of a pedicle, for the protrusion of 

 which a minute foramen is sometimes observed in the beak. From the 

 sides of this process, above the junction of the teeth of the opposite 

 valve, and at the point where it bends upwards, originate the brachial 

 processes which support the spires. A deep cavity beneath the cardinal 

 process extends to the dorsal beak, from which originates a thin 



