492 CAMBRIAN BRACHIOPODA. 



LiNGULELLA DESIDERATA (Walcott). 

 Plate XX, figures 4, 4a-c, 5, 5a-j. 



Obolus (Lingulella) desideratus Walcott, 1898, Proc. U. S. Nat. Mus., vol. 21, pp. 399-400. (Described and discussed 

 as a new species.) 



Oholus {Lingulella) desideratus Walcott (in part), 1899, Men. U. S. Geol. Survey, vol. 32, pt. 2, pp. 445^446, PL LX, 

 fig. 2 (not 2a). (Described essentially as below, and discussed. The specimen represented by fig. 2 is redrawn 

 in this monograph, PL XX, fig. 4. The specimen represented by fig. 2a is referred in this monograph to Oholus 

 rotundatus, PL XX, fig. 2e.) 



Dicellovius nanus Walcott (in part) [not Meek and Hayden], 1899, idem, p. 447, PI. LX, fig. 3 (not figs. 3a-d). 

 (Mentioned. The specimen represented by fig. 3 is redrawn in this monograph, PL XX, fig. 5j. The speci- 

 mens represented by figs. 3a-d are referred in this monograph to Dicellomus nanv^.) 



Shell small, subovate, with the ventral valve obtusely acuminate, .and the dorsal valve 

 broadly ovate. Valves are strongly convex, with the ventral valve fully as much so as the 

 dorsal. There is a slight variation in the outline of the valves, some being slightly more 

 rounded posteriorly than others. 



The surface of the shell is marked by fine, concentric lines of gi'owth, and between them 

 very fine, slightly irregular striae; a few rather narrow indistinct undulations radiate from 

 the umbo toward the front and lateral margins; when the outer shell is partly exfoliated the 

 outer surface of the inner layer is marked by very fine indistinct radiating strias; there are a 

 few traces of small, scattered pits or punctse on the inner surface. The shell is thin and formed 

 of an outer layer and one or more inner layers cr lamellae. 



The average length of the ventral valve is about 4 mm.; width, 3 mm. A dorsal valve 

 3.5 mm. long has a width of 3 mm. 



A cast of the interior of a ventral valve shows an area of medium length, divided midway 

 , by a narrow, clearly defined pedicle groove. The area of the dorsal valve is short. Nothing 

 is known of the interior of the ventral valve, but in a cast of a dorsal valve -there are traces 

 of the main vascular smuses, central median septum, and the central muscle scars. 



Ohservations. — This species may be compared with the Middle Cambrian Lingulella ferru- 

 ginea Salter of the Atlantic basin faunas, and L. similis (Walcott), of the Black HUls, upper 

 Mississippi Valley, and Appalachian faunas. Lingulella similis is more convex and narrower, 

 and L. manticula (White) is considerably more acuminate. Compared with the Rocky Moun- 

 tain species it is intermediate between L. manticula and L. rotunda (Matthew) . It may also be 

 compared with L. granvillensis Walcott of the Olenellus fauna of eastern New York, and L. 

 iole (Billings) of the Lower Ordovician fauna of Newfoundland. It is distinguished from 

 L. granvillensis by its more regularly rounded anterior margin. 



The specimens represented by Plate XX, figures 4b, 4c, and 5j, are broader proportionately 

 than the typical specimens represented by Plate XX, figures 4 and 4a, but they occur at the 

 same geological horizon in the Eureka district, Nevada. What appears to be the same, or a 

 closely related species, occurs in the upper beds of the Secret Canyon shale in the Eureka dis- 

 trict, Nevada. 



A form that appears to be identical occurs ia considerable abundance m Alabama and 

 Tennessee. The similarity in form between the specimens from Wyoming and Nevada, and 

 those from Alabama and Tennessee is shown by Plate XX, figures 4, 4a-c, of the t3^pical 

 specimens, and Plate XX, figures 5, 5a-i, of their representatives in the Appalachian region. 

 This similarity is even more striking when the specimens are compared directly with one another, 

 and some allowance made for the fact that the Appalachian specimens have all been more or 

 less compressed. 



A small shell that appears to be identical with this species qccurs in the red sandstone 

 and argillaceous shale of the Lower Ordovician of Colorado. The specimens from Trout Creek, 

 below Bergen Park, are much like those from the Gallatin Range in Montana and the same 

 species of BUlingseUa is associated with them. At Cement Creek, 10 miles (16.1 Ian.) south- 

 east of Crested Butte, Colorado, the shells occur in a fine conglomerate and coarse sandstone 



