HAMBACH ANATOMY OF THE GENUS PENTREMITES, ETC. 153 



Pentremites melo^ where the ba.se portion of the deltoid piece is 

 so broad, and the shell so very thin, that it never appears as if 

 the central opening was surrounded by only five instead of ten 

 distinct openings, excluding ihe lateral one for the anus. We will 

 have, therefore, twelve openings on the summit in all true Pen- 

 tremites, i.e. one central for the mouth, one lateral for the anus, 

 and ten acting in a double capacity ; that is, as spiracle and also 

 as ovarian outlet. The alimentary canal, which from analogy I 

 suppose has occupied the rest of the interior cavity, has not been 

 observed so far, though I have often found large tubular frag- 

 ments in cross-sections, but too insufficient to justify any definite 

 opinion. 



If we now, in short, compare the different subgenera of Pen- 

 tremites — that is, Granatocrimcs and Troostocrinus — in order 

 to determine the justification of having them separated from Pen- 

 tre?t?ttes^ we will find that Gratiatocrinus has a globose or ovoid 

 form of the calyx, with very small or linear ambulacral fields 

 extending almost over the entire surface of the body, with, gene- 

 rally, a small pelvis more or less depressed. Troostocri77us has a 

 pear-shaped body ; base portion generally more or less expanded, 

 with a triangular or cloverleaf-shaped depression or indentation ; 

 ambulacral fields narrow and lineary, similar to those of Gra- 

 natocrinus ; and a triangular articulating surface for the column. 

 But all these peculiarities are met with in different degrees in 

 true Pentremites, and are so common that it is often very difficult 

 to say where the dividing line of two species should be ; for we 

 have true Pentremites with a globose body and a depressed pel- 

 vis and somewhat narrow ambulacral fields, as well as clavate- 

 shaped forms, with lineary or broad ambulacral fields with three 

 edges flattened on the base poi'tion ; the indications of the trian- 

 gular depression ; also with the triangular articulating surface for 

 the column. The deltoid pieces are subject to all these variations 

 as well as all the other parts forming the calyx. For instance, in 

 Pentremites Sayi, as already stated, where the apex of the del- 

 toid piece is so expanded as to form the most essential part of the 

 calyx ; in P. melo it is very small, and shows beautifully the ar- 

 rangement of the ovarian openings; in P. Norivoodi its rostrum 

 is horizontally so expanded that Nature found it necessary to per- 



