CARBONIFEROrS AGE. 19 



generic group, to which Koenig gave the name Trigonotreta, subsequently adopted 

 by Prof. Bronn, and by Prof. King.^ 



As here defined and typified, it will be seen that this genus includes two rather 

 marked sections, with one or more less distinctly defined groups, which may be 

 characterized as follows : — 



1. Spirifer (proper), Sowekbt, = Ci/rtia, Dalman, and others. 



Shell trigonal, nearly always eostate or striate ; hinge generally a little less than the breadth of the valves, 

 more or less angular at the extremities ; ventral valve very prominent or pyramidal ; beak straight or a 

 little curved ; area large and triangular ; foramen closed by a false deltidium, generally pierced by a small 

 round or oval aperture near the apex of the beak ; dorsal valve comparatively compressed, semicircular. 



2. Trigonotreta, Kcenig, = Spirifer, of most authors. 



Shell usually with both valves convex, marked as in the foregoing ; lateral extremities generally angular, 

 often acutely so ; hinge line usually extended. Both valves with beaks more or less incurved ; area 

 variable in size, but rarely if ever so extravagantly developed as in some of the typical Spirifers ; foramen 

 open or more or less closed by a false deltidium without a perforation. 



Example. — Spirifer striatus, Sowerby. 



3. Martinia, McCoy, = Amboccelia, Hall. 



Shell subglobose, or transversely or longitudinally oval, smooth or rarely with compressed rounded costse ; 

 often beset with minute hair-like spines. Hinge short, scarcely ever equalling the greatest transverse 

 diameter of the valves ; lateral margins rounded, or rarely obtusely angular ; beak of ventral valve 

 incurved ; cardinal area comparatively small ; foramen, and false deltidium as in Trigonotreta. 



Type. — Spirifer glaber, Sowekby. 



The genus Spiriferina of D'Orbigny, is also often considered a subgenus under 

 Spirifer. The coincidence, however, of a punctate structure in these shells, with 

 the presence of a prominent mesial septum in the ventral valve, would seem to 

 indicate important difi'erences in the structure of the animal, such as would warrant 

 its separation as a distinct genus.^ So also with Cyrtina of Davidson, which was 

 formerly included Avith the forms we here regard as the typical Spirifers, imder the 

 name Cyrtia. Mr. Davidson has, however, very properly separated these shells, on 

 account of their punctate structure, and the peculiar development of the dental 

 laminae of the ventral valve, which are produced inwards, and coalesce into a single 

 mesial septum, extending from the extremity of the beak, almost to the anterior 

 margin, thus giving the interior more the appearance of Pentamerus than Spirifer. 

 As Mr. Billings has, however, discovered internal spiral appendages in one of these 

 shells, and they all possess a well-developed cardinal area, we cannot doubt the 

 propriety of including them in the SpiriferidcB, though we think, as above stated, 

 that they should constitute a distinct genus from all the other groups. 



As thus freed from the punctate species, the typical Spirifers (^ Cyrtia, Dalman) 

 are found to pass by such insensible gradations into the Trigonotreta group (= the 



' In his valuable ^yol'k on the Permian fossils of England, Prof. King regarded Spirifer cuspidatus 

 as the type of the genus, and adopted Koenig's name Trigonotreta for the shells generally considered 

 typical Spirifers. It is said, however, that he subsequently changed his views in regard to the type 

 of Spirifer. 



° I avail myself of this opportunity to refer to the genus Spiriferina, a species described by me 

 under the name of Spirifera pulchra (Proceed. Acad. Nat. Sci. July, 1860, p. 310), brought by 

 Capt. Simpson from Nevada Territory. Sections of it show very distinctly a coarsely punctate, very 

 finely fibrous structure ; while casts of the interior exhibit a deep slit left by the mesial septum in the 

 beak of the ventral valve — F. B. M, 



