DEVONIAN FOSSILS. 503 



squamose. Beak of the ventral valve small, not much incurved; area small. 

 Anterior margin distinctly but not strongly sinuate. 



There can be little doubt that this is the same form for which Miller 

 proposed the name Atrypa missouriensis, and which had been previously 

 referred to by Meek as Atrypa reticularis and by Walcott as Atrypa desqua- 

 mata (loc. cit). 



A. missouriensis is described from Middle Devonian rocks (probably 

 Hamilton age), and the type locality is 3 miles from Otterville and 17 

 miles west of Sedalia, Missouri. A form, probably referable to Miller's 

 species, is found at Fulton, Missouri, in rocks of Hamilton age, and with 

 this the material from the Yellowstone National Park is very closely 

 allied, perhaps specifically identical. It is possible that the finely striated 

 variety of A. reticularis, mentioned by Walcott 1 and said to resemble a 

 variety from the Hamilton and Chemung groups in Iowa, may also be 

 placed in the list of synonyms. 



A. missouriensis occurs in considerable abundance at several localities 

 in the Yellowstone Park, but it is rarely found associated with Atrypa 

 reticularis, which is known from the same region. The two forms are thus 

 distinct in distribution as well as intrinsic character, when this region 

 alone is contemplated, and the natural tendency is to refer them to 

 different species, but in view of the almost universal distribution of A. 

 reticularis, and its equally extensive range of variation, perhaps a varietal 

 distinction is all that is warranted. Only a monographer will be com- 

 petent to determine specific limitations in this protean type. 



A. desquamata in this country is nothing more than A. reticularis 

 with an erect beak, area, and unconcealed foramen; and the same appears 

 to be equally true of the European forms. This character or group of 

 characters seems scarcely of specific value, but in any case the same 

 peculiarities of surface, etc., which distinguish A. missouriensis from A. 

 reticularis serve to differentiate it from A. desquamata also. Although it 

 is a mature form, A. missouriensis is characterized by neologic traits, and 

 agrees very closely with young examples of A. desquamata as figured by 

 Davidson. 2 Still, full-grown shells are the only ones with which we are 

 justified in comparing it. 



»Mon. U. S. Gcol. Surv., Vol. VIII, 1884, p. 150. 



2 British Fossil Brachiopoda, Vol. Ill, Part VI, PI. XI, figs. 6, 6a. 



