HAMBACH ANATOMY OF THE GENUS PENTREMITES, ETC. 149 



acute point of the deltoid edge becomes a projecting rostrum 

 above the summit of the calyx, whereas in others, as for instance 

 Pentremites Norwoodi^ the expansion is lateral and horizontal, 

 in which case it is perforated in the centre to afford a passage for 

 the ovalum. 



The A?nbulacral field (see Plate a, Fig. 6 & 7). — The am- 

 bulacral field, i.e. the calcareous portion of it, is composed of 

 three parts and the pinnule : first, the lancet piece ; second, the 

 poral pieces ; third, the zigzag plicated integument covering the 

 former. 



A. The lancet piece, which varies very much according to the 

 dimensions of the ambulacral field, from a regular lancet-shaped 

 piece to a mere linear lamella, closes the ambulacral opening, 

 with the exception of a three-cornered furrow on each side. 



Its anterior side is smooth and slightly convex, whereas the 

 posterior is concave, semilunar, and grooved in its whole length 

 for the reception of some duct or vessel. On this side we find, 

 on its upper portion, a little process, which is concave laterally 

 and three-cornered at the apex, by which it connects with the 

 annuhis centralis ; its lateral concave furrow forms, in connec- 

 tion with the con-esponding portion of the deltoid piece, the ova- 

 rian aperture. In the majority of the species, or at least in the 

 typical ones, as Pentremites fior calls ^ sulcatus^ pyriformis^ etc., 

 the width of the lancet piece is half as great as that of the ambu- 

 lacral field, being also pierced through the centre, in its whole 

 length, by a very fine canal, which led Mr. Rofe* to suppose 

 that it was composed of two pieces. Numerous examinations of 

 cross-sections have convinced me, however, that this is not the 

 case, as I have, in entirely well-preserved specimens, found this 

 minute canal right in the centre (see Plate a. Fig. 9) ; also, in 

 some specimens, either near the anterior or posterior surface. In 

 specimens in which the opening is in the centre, it may be seen 

 laid open externally if the covering layer is removed by process 

 of weathering or acid, though in the majority of cases it will pro- 

 bably be found entirely obliterated and therefore invisible. 



B. Poral pieces. These pieces which, in typical cases, are very 



* GeoL Mag., vol. ii. p. 249. 



