THE BRACHIOPOD SHELL. 301 



such as that of Micromitra {IpUdeUa) fannula (PL IV) and Botsfordia cselata (PI. LIX), were 

 fully developed in Lower Cambrian time. 



I have given much attention to the question of the value of surface ornamentation for the 

 purposes of systematic classification among the Cambrian brachiopods, and have concluded 

 that only in cases where a distinctive surface is persistent for a considerable period can it be 

 considered of more than specific value. In one instance I have given the type of surface found 

 on Oholus (Westonia) aurora (PL XLVI, fig. Ih) a subgeneric value, but its wide range of varia- 

 tion makes me doubtful of its utility. 



ATREMATA. 



So far as known, the outer surface of Rusiella, the most primitive type of brachiopod known 

 (PL I, figs. 1, la-e), is marked only by concentric growth lines. Among other genera referred to 

 the PaterinidjB, VoIbortUa (PL I, figs. 6, 6a-e) has only concentric growth lines on the outer 

 surface, although it has numerous radiating lines on the inner laminations of the shell. On 

 the outer surface in the genera MicJcioitzia (PL VI) and Helmersenia (PL LXIII, figs. 7, 7a, and 

 7e) are found concentric and radiating lines and small tubercles (apparently rudimentary 

 spines) that are ornamental in their arrangement and effect, but it is in Micromitra that the 

 greatest variety and most .liighly ornamented surfaces appear. Species that have a simple type 

 of surface, marked only by concentric fines of growth, are grouped under the subgenus Paterina 

 (PL II), while those having an ornate surface (PL IV) are included in the subgenus Iphidella. 

 Transition forms, intermediate in surface ornamentation between the two extreme types, are 

 referred to Micromitra, since the type species, M. sculptilis, has this intermediate form of 

 surface. The species Micromitra {Paterina) pealei (PL III, figs. 3, 3a-e) approaches closely to 

 Micromitra sculptilis (PL III, figs. 5, 5a, and 5e) but is not quite so highly ornamented. Micro- 

 mitra (Iphidella) pannula and its varieties (PL IV) have a liighly ornate surface, and the species 

 has both wide geographic distribution and stratigraphic range. It is abundant in the Middle 

 Cambrian and it also occurs in the upper portion of the Lower Cambrian section in Nevada. 

 The oldest known brachiopod from British Columbia, Micromitra {IpMdella) louise, which 

 occurs 3,090 feet down in the Lower Cambrian terrane, has an ornamented surface much like 

 that of M. (7.) nyssa (PL III, fig. 9), wliich occurs at the smnmit of the Lower Cambrian in the 

 same stratigraphic section. 



None of the Obohdse of the Lower Cambrian is known to have an ornamented surface, but 

 in the Middle Cambrian many species show, in addition to the concentric strise and lines of 

 growth and the radiating stride and ridges, a variously pitted and granulated surface primarily 

 formed by the crossing of the radiating and concentric ridges by oblique undulating ridges. 



What I have called the Westonia type of surface (Pis. XLVI and XLVII) is the most 

 common. A variety of this type where the fines are transverse is more rarely seen (PL XLIX, 

 fig. If and Ig). The Westonia surface is not confined to Oholus (Westonia), as it occurs among the 

 Neotremata in the genera Obolella (Glyptias), Botsfordia, Acrothele, Acrotreta, and AcroiJiyra. 

 (See description of OhoJus (Westonia), p. 450.) It is also present on Ordovician and later brachio- 

 pods and it is known on Lucinia divaricator and other pelecypods. [See Winchell and Schuchert, 

 1895, pp. 345-346.] 



In Oholus (Acritis) (PL XIII, fig. 3) the irregular, undulating, and more or less inosculating 

 concentric ridges suggest one of the rudimentary characters of some types of the Westonia surface, 

 but their origin was probably different. The surface oi Lingula cincta Barrande [1879b, PL CV, 

 figs. lA, 2A, and 2f], from Etage E (Ordovician), is much like that of Oholus (Acritis) anti- 

 quissimus, but it is probably an illustration of the persistence of a form of surface sculpture 

 through several geologic formations and a corresponding period of time. 



The surface of Lingula eximia Barrande [1879b, PL CV, block 1] is highly ornate. The 

 figure is introduced on Plate XXXII, fig. 3, as a possible type that may be found to have existed 

 in late Cambrian time. Oholus (Miclcmtzella) siluricus (PL XV, figs, lb and Ic) has fine crenula- 

 tions on the front side of certain concentric ridges, a type of ornamentation that is unknown 

 among the Cambrian brachiopods except in this one Upper Cambrian species. 



