28 PALEONTOLOGY OF THE UPPER MISSOURI. 



1. Pterinilnae (or Pterinia group). 



Cartilage apparently occupying a series of linear furrows, ranging more or less nearly parallel to the cardinal 



margin, in a usually broad, flattened cardinal facet or area. Anterior muscular scar sometimes moderately 



developed and deep. 

 Includes Pterinia, MyaUna, AmbonycMa, and probably Actlnoilesma, Grypktrkyndnu,' Eurtjdesma, and several 



undefined Paleozoic groups. A part of the species referred to the genus Megambonia (3f. avicu/aides, M. 



lamellosa, &c., HALL), will probably be found to belong to this subfamily, if not indeed to the genus Pterinia, 



while the typical species appear to belong to the family Arcidse. 



2. Pteriiiiae Cor Aviculinas). 



Cartilage mainly or entirely confined to a single more or less defined depression or cavity behind the beaks. 

 Anterior muscular impression very small. 



Includes Pteroperna, Pteria (or Avicula), 3fargaritifera, Malleus, Aucella and Ettmicrotis. 1 The following 

 extinct genera also probably belong here, viz. : Monotis, Halobia, Pteronites and PouidoHoaia, with appa- 

 rently some undescribed fossil genera. 



3. Melilliiiae (Perna or Isoynomon group). 



Cartilage divided and distributed along the hinge in a series of furrows crossing the cardinal area at right 



angles to the hinge line. Anterior muscular scar generally very small. 

 Includes Crenatula, Melina (= Perna, BKUG. not ADANSON), BakevtUia, Gervillia, Inoceramus and Pulvinites. 



The first two of these sections seem to be more nearly related, in some respects, 

 to each other, than either is to the third ; and it is not improbable that they will 

 be found connected by a few Triassic and Jurassic forms presenting intermediate 

 characters, when the hinge and interior of a greater number of species are known. 

 The Jurassic genus Pteroperna, for instance, has hinge teeth analogous to those of 

 Pterinia, with apparently a cavity or depression for a cartilage, similar to that of 

 Avicula. Such exceptional cases, however, cannot be urged as a reason for not 

 admitting the convenience of sections or intermediate groups between families and 

 genera, for it is highly probable that if AVC knew all the characters of all the species 

 that ever existed, from the beginning of animal life to the present epoch inclusive, 

 we would find all our groups blending imperceptibly together, or at least very far 

 from being so sharply defined as they appear in works on natural history. 



The Pterinia group, or subfamily, probably includes most of the Palaeozoic 

 species usually referred to Avicula, especially those from the Silurian and Devonian 

 rocks. Indeed, we very much doubt the existence, during the deposition of the 

 Palaeozoic rocks, of true Aviculas, as that genus is known to Conchologists, and 

 typified by the existing A. Jiirundo. At any rate, we have never seen a specimen, 

 nor can we remember a figure, of any species showing the hinge of a true Avicula, 

 from any of our American Palaeozoic formations. All the Silurian and Devonian 

 species .with which we are acquainted, the hinge of which has been seen, appear to 

 want the cartilage cavity of the modern Aviculas, and have the striated hinge facet, 

 or the oblique hinge teeth (one or the other or both) of Pterinia, more or less dis- 

 tinctly marked. In addition to this, most of the Silurian and Devonian, as well as 

 many of the Carboniferous species, the hinge of which is unknown, present more 

 the external appearances of the European species figured by Goldfuss and others, 

 in which the internal characters of Pterinia are known to exist. 



Prof. McCoy some time since referred three of our American Paleozoic species 

 A. demissa, A. pleuraptera and A. subfalcata to Pterinia ; and the figures of A. 



1 See American Journal Sci. and Arts (2d ser.), XXXVII, March, 1864, 217. s Ib. 21G. 



