472 CAMBRIAN BEACHIOPODA. 



the case with the position of the muscle scars of the ventral valve to wliicli Matthew refers. 

 The central muscles of the ventral valve of Oholus vary greatly in the position of the points of 

 attachment ; tliis is shown by Plate VII, figures 1-13, for Oholus apoUinis. The scars of the central 

 muscles of Lingulella are rarely preserved so that they can all be identified; in one species, 

 Lingulella acutangula (PL XVII, figs. Ic and Ig), they appear to be arranged as in Oholus 

 apollinis. The same is true for Oholus (Westonia) escasoni (Matthew) (PL XLIX, fig. laa). 



The genus Schmidtia Volborth is made a subgenus of Oholus by Mickwitz [1896, p. 25]. 

 A comparison of typical specimens of Oholus {Schmidtia) celatus Volborth and Lingulella 

 davisi (McCoy) leads to the view that Schmidtia is identical with Lingulella except that Lin^' 

 gulella is a radially striated shell and Schmidtia is not. I did not fuUy appreciate tliis distinc- 

 tion when writing a preliminary notice [1898, p. 392] of the study of Lingulella and hence 

 referred to Schmidtia as a synonym of Lingulella. 



Any extended comparison with Oholella is unnecessary, as it is a thick, round-ovate shell 

 with a cardinal area and pedicle opening unlike Lingulella or any of the subgenera of Oholus. 

 Linguloholus has the same arrangement of muscle scars as Lingulella, but differs in the greater 

 tliickness of the shell in both valves. Lingulepis is scarcely more than a very acuminate form 

 of Lingulella with the area and visceral ca^aty of the ventral valve modified in shape to meet 

 the extreme elongation of the posterior portion of the valve. 



That Lingulella is closely related to Lingula is seen at once by comparing the markings 

 on the interior of the two shells. 



Under observations on Oholus (p. 375) the differences between Oholus and Lingula have 

 been stated. Lingulella differs from Lingula in the same manner as Oholus in having its visceral 

 area continued back to the posterior margin of the interior of the valves, but it has the same 

 general form as Lingula. Without the interior of the valves for comparison it would be very 

 difficult to determine that many of the Cambrian species were not true species of Lingula. 



Matthew [1903, p. 105] refers a group of small shells heretofore placed under Lingulella 

 to the Ordovician genus Leptoholus of Hall. He says : 



Such forms have usually been included in Lingulella, but in addition to their minute size they are separated 

 from that genus by important characters. 



These little shells have thin test, weak umbones; vascular trunks are situated near the lateral margins, and both 

 the central and lateral muscles in both valves are advanced far toward the front of the valves. 



As a result of the examination of the types of the Ordovician species of Leptoholus I find 

 that they have quite strong, tliick shells in proportion to their size, quite as much so as the 

 young shells of species of Lingulella that acquire tliickened shells as they attain their fuU 

 growth. The vascular trunks are well out toward the margin, but this feature also occurs in 

 large forms of Lingulella, as indicated in the table under Oholus (p. 377). The position of the 

 central and lateral muscle scars is also too variable a character to afford a basis for generic 

 reference. Leptoholus is marked by a peculiar visceral area in the ventral valve imknown in 

 any Cambrian shell, and the bifurcation of the strongly marked ridges of the path of advance 

 of the central and lateral muscle scars in the dorsal valve is a distinguisliing character not 

 found in the Cambrian shells referred to Leptoholus. The Cambrian forms referred to Lepto- 

 holus by Matthew [1903] are: L. atavus, L. atavus insulx, L. atavus tritavus, L. collicia, L. 

 collicia collis, L. jlumenis, L. gemmulus, L. cf . linguloides, and L. torrentis. 



If the above species are included in a distinct genus, several others should go with them, 

 notably: Lingulella ferruginea Salter (PL XXIX), L. lepis (Salter) (PL XXXI), and L. cania 

 (Walcott) (PL XXXV). 



One of the oldest species known of the genus Lingulella, Lingulella schucherti (Walcott) 

 (Pl.XXI, fig. 6), is in all respects, so far as can be determined from the material, a true Lingu- 

 lella. The shell is phosphatic, relatively tlnn, elongate, resembling in tins respect Lingulella 

 acutangula (Roemer) (PL XVII). From this it appears that we have in the Olenellus fauna 

 of the Lower Cambrian, associated with Oholella, a typical representation of the genus Lingu- 

 lella. This association shows that the differentiation in the group of brachiopods represented 

 by Lingula and Oholella, or the Lingulidse, had advanced a long way before the period in Lower 



