148 TRANS. ST. LOUIS ACAD. SCIENCE. 



The external edges are flattened to form a surface for connection 

 with the adjoining pieces, and the upper points of the prongs 

 slope inwardly for connection with the lower lateral processi of 

 the deltoid pieces. Their external surface is plain, or, when 

 ornamented, the ornamentation is similar to that of the pelvis. 

 Two of the fork pieces rest, each on one of the pentagonal plates : 

 while the three others rest, each on two adjoining ones. 



The Deltoid pieces. — The deltoid pieces, which may be re- 

 garded as keystones in the arch forming the roof of the calyx, 

 may be divided into two parts, on account of their structure (see 

 Plate a, Fig. 4 & 5 a 3), i.e. one external lateral, from which the 

 name is derived, and one internal horizontal which enters into 

 the composition of the central orifice and in part of its surround- 

 ing openings. 



The external lateral part, when present, has, as the name 

 denotes, a more or less deltoid or arrow-head shape, with two 

 unequal angles. It varies very much in size, fiom a hardly 

 perceptible piece to the most essential part of the calyx, as in 

 Pentremites Sayi. In some species its external surface is orna- 

 mented. The most obtuse of the angles is directed downward, 

 and has (in the typical species) on its lower oblique margins two 

 processi running obliquely and inwardly, which are hidden ex- 

 ternally by the superior points of the fork pieces. 



The internal horizontal part, i.e. the main and base portion of 

 the body, because it is never missing, is lamellar running from 

 the interior outwards, where its apex suddenly developes into the 

 deltoid expanded surface ; or, when this is absent, the apex re- 

 ceives a three-cornered incision for the reception of the acute 

 points of the fork pieces. The interior portion is expanded on 

 each side and concave, thus forming the central orifice (s. annu- 

 lus centralis., Plate a, Fig. 8), as well as, in connection with the 

 lancet pieces, the surrounding openings. Its position is a little 

 oblique and therefore the external diameter is less than the inter- 

 nal. It is thicicer than it is wide, and has a lower concave edge, 

 thus giving to the calyx its dome shape. On each side it has an 

 oblique plicated lamella sloping from the centre to the deltoid 

 expansion for the support of the hydrospiric plicas and ovarian 

 tubes. The upper edge is convex, so that, in some species, the 



