28 UNITED STATES GEOLOGICAL SURVEY OF TIIE TERRITORIES- 



Locality and position. — Sage Creek; from the upper part of the Fort 

 Pierre group ; also, from the Fox Hills group, or formation No. 5, at Fox 

 Hills, and on Moreau River. 



HETEROMYAR1A.* 



PTERIID^. 



Genus PTERIA, Scopoli. 



Synon.—Avicula, Klein (1753), Ostrac, 120.— Brug. (1792), Encyc. Me'th., pi. 177.— Lam. (1799), Prodi-., 82; 



and (1-801), Syst. An. 134.— Blainv. (1816), Diet. Sci.Nat., Ill, Suppl., 138.— Schum. 



(1817), Ess., 137. — Okeu (1S35), Allg. Natgsch., V, 300; and numerous others, down to 



the present time. 

 Pteria, Scopoli (1777), Iutrod. ad Hist. Nat., 397.— Gray (1847), Zool. Proc., 199.— Meek (1864), 



Smithsonian Check-List N. Am. Cret. Fossils, 8 ; and Palaiont. Upper Mo., 28. 

 Anonica, Okeu (1815). Handb. d. Zool., * * ; and (182.1), Natgsch. d. Schulen, 652. 

 Oxytoma, Meek (1604), Smithsonian Check-List N. Am. Cret. Fossils, 39 (as a subgenus). 

 - Pseudoptera, Meek (1873), Hayden's Ann. Report IT. S. Geol. Survey of the Territories, 489 (as a 



subgenus). 

 Electroma, Stoliczka (1871), Pakcout. Iudica, III, 391 (as subgenus). 



Etym. — rrrepov, a wing. 

 Type. — Mytilus hirundo, Linn. 



Shell fragile, inequivalve, the left valve being more convex than the 

 other ; general outline usually obliquely subtrigonal ; cardinal margin straight, 

 generally provided with a narrow, flattened or concave, gaping area, and ter- 

 minating in a more or less produced anterior and posterior wing, the latter of 

 which is always larger than the other ; ligament partly internal; hinge usually 

 with a single cardinal tooth under the beaks in each valve, and sometimes a 



o 



* I follow Dr. Gray, Dr. Broun, and my frieud Dr. Gill, in the use of this convenient group between 

 the Honomyaria and Dimyaria. It must be admitted, however, that when we undertake to classify the 

 whole extensive series of bivalves, including all of the known extinct types of all ages, as well as the 

 existing forms, wo find, as in many other cases, that the limits of these primary divisions are often much 

 less sharply defined than among existing mollusks only. Take, for instance, the extinct genus Bakemllia, 

 with often the exact form and alations of Pteria [== Avicula), and even a series of large cartilage-pits as 

 in Gervillia, Melina (= Perna, Brug.), Inoeeramus, &c. (by all referred to the Aviculidcu, most usually 

 considered by eonchologists a monomyariau family); yet we see that in some of the species, the ante- 

 rior muscular scar is so large, that even Professor King, a very learned naturalist, placed it in the 

 Dimyaria : while Dr. Stoliczka, actually refers it to the dimyarian family Arcidai. On the other band, 

 Dr. Geinitz and Mr. Woodward were so much impressed with the close relations of this genus (Bakevellia) 

 to Gervillia, of the AricuVuUr, that they both (wrongly, as I think) include it as only a section of that 

 genus. In another direction, among ancient types, the AvicuKdw shade off toward the nionomyarian 

 family Peclinidas. 



Various other examples might be mentioned among palaeozoic shells that seem, as it were, almost to 

 break down the distinctions between the three primary divisions Monomyaria, Heteromyaria, and Dimy- 

 aria : but the above will perhaps be sufficient to illustrate the fact. 



Dr. Gray called the intermediate section the Pogonopoda, and in 1*42, included in it, along with the 

 Mytilidai, the Tridacnidos, the Aviculidoa and the Pinnidce ; while iu 1849, Dr. Broun called it the Hetero- 

 mija, using it, 1 believe, rather as a section of the Dimyaria, and included in it the Mytilidce, the Avicu- 

 lidos and Tridaemdos. Dr. Gill, however, views it as an order : and (in bis excellent " Arrangement of the 

 Fain lie.-, of Mollosks," published by the Smithsonian Institution in 1871), only includes iu it the Mylilida , 

 while lie writes the name Heteromyaria. 



