304 CAMBEIAN BRACHIOPODA. 



(PI. XXII, figs. 1 and lb), Lingulella nathorsti (PL XXXI, figs. 1 and la), and lAngulella schu- 

 cJierti (PI. XXI, fig. 6), all of which are from the Lower Cambrian. Lingulella schucherti is 

 probably the oldest known, species, unless it be Delgadella lusitanica. In Dicellomus the cardinal 

 area of the ventral valve (PI. LII, figs. Ic and Id) is not unlike that of Oholus, except that short, 

 teeth (PI. LII, fig. Id) appear to have been developed, and that on the cardinal area of the 

 dorsal valve (PI. LII, fig. li) there is a socket-like groove on each side for the reception of the 

 projection in the ventral valve. On other specimens of the area of the dorsal valve there 

 are many minor depressions or crenulations (PL LII, fig. Ij), but no defined groove or socket. 

 On the dorsal valve of another species the cardinal area (PL LIII, fig. 1) is not unlike that of 

 the dorsal valve of Oholus. There is considerable variation in the details of form and size of the 

 vertical cardinal areas of the various species of Oholus, its subgenera, and Lingulella, but it is 

 all witliin the typical Obolus-like cardiaal area as seen in Oholus apollinis (PL VII). A curious 

 individual developnaent or extension of the area is shown in some specimens of Oholus namouna 

 (PL VIII, fig. 2a) and 0. rhea (PL IX, fig. 1), where the posterior margin of the mantle appears 

 to have been drawn farther forward inwardly and a thin deposit of shell made, so as to carry 

 the front margin ,of the cardinal area far in advance of its usual position in the species of the 

 genus. On^icia, one of the oldest genera (PL L), the area is as strongly defined as it is in 

 Oholus and its subgenera, and teeth or articulate processes are suggested. In Elkania (PL LI, 

 figs. 1 and la) the cardinal area so merges into the thickened platform that it is not readily 

 separable from it- I have referred to this as the internal cardinal area, or the area that is 

 inclined inward from the plane of the margins of the valves and united by shell deposit to the 

 interior of the shell. In Neoholus the internal cardinal area of the dorsal valve extends forward 

 as a semicircular shelf (PL I, fig. 4b). It is a modified cardinal area on the plane of the ma^ipins 

 of the valves, but in the adult it does not extend over the rounded posterior margin of the valve. 

 The area of the ventral valve is unknown, but from the presence of an open delthyrium (PL I, 

 fig. 4') it is not probable that a well-defined cardinal area exists. 



In the dorsal valve of Lingulella granvillensis (PL XXII, fig. Id), Lingulella helena (PL XXIV, 

 fig. 3c), Oholus (Westonia) euglyphus (PL XL VIII, fig. le) there is a strong undercut or opening 

 beneath the area adjoining the pedicle furrow. This also occurs in the ventral valve of the 

 last-mentioned species (PL XL VIII, figs. Ic and Id) and in many species of Oholus and its sub- 

 genera and in Lingulella. In other species the area may be thickened so as practically to 

 close up the space between it and the inside of the valve (PL VII) . 



In Eutorgina (PL V) the cardinal areas are in a very rudimentary condition, and com- 

 parable with the rudimentary cardinal areas of Micromitra (Pis. II, III, and IV). The general 

 form and structure of Kutorgina, taken in- connection with the rudimentary cardinal areas 

 beneath the beaks of the valve, give the areas a significance that those of Micromitra do not 

 possess. It is highly probable that a number of intermediate forms, in which the cardinal 

 areas gradually obtained their more mature character, were developed between the type from 

 which Kutorgina was derived and Nisusia and Billingsella, but nothing is known of such inter- 

 mediate forms in the Lower Cambrian. Schuchertina (PL LI, figs. 6, 6a-m) appears to have 

 been descendant from some atrematous type that may also have been the ancestral stock of 

 Kutorgina and the forms that passed into Nisusia and Billingsella. Schuchertina is a primitive 

 protrematous genus still preserving strong atrematous characters — or vice versa, according to 



one's point of view. 



NEOTBEMATA. 



The cardinal areas of the valves of Oholella (Pis. LIV and LV) of the Lower Cambrian are 

 essentially the same in appearance as those of Oholus, except that they are not usually so strongly 

 developed. They may also be compared with the cardinal areas of Bicia (PL L), especially 

 the projections on the inner angle adjoining the pedicle furrow. The narrow, thickened cardi- 

 nal margin in the ventral valve of Oholella does not clearty show from the interior view, although 

 such specimens as that illustrated on Plate LIV, figure 2h, indicate that the area is not so promi- 

 nent as in the ancestral form. Tliis is more clearly shown on Plate LIV, figure 2g. In 



