THE BRACHIOPOD SHELL. 303 



they are strong on the inner surface and less marked on the outer surface. Other species of 

 Kutorgina (PI. V) have a smooth, or only concentrically lined, surface. Schuchertina camhria 

 (PI. LI, figs. 6, 6a-m) has a fine series of internal radiating striae and ridges, but the exterior 

 surface is without them. 



Among the Protremata the radiating ridges are present in most genera, although some 

 species of a genus may not have them. In the Billingsellinse a few species with smooth shells 

 occur (PI. LXXXVII, figs. 1, la-e, 7, 7a, 5, and 5a), but the greater number have both interior 

 and exterior radiating ridges (Pis. LXXXVII-XC). Among the Nisusiinse and EoorthiuEe 

 all the species show more or less of external radiating lines and ridges; sometimes the young 

 shells are nearly smooth (PI. XCVIII, figs. If, 2b, and 5), but this in most cases is at a very 

 young stage of growth. The Syntrophiidse show variation from the smooth adult shells of 

 Syntrophia calcifera (PI. CIV, figs. 1, la-i) to the strongly costate shells of Huenella texana 

 (PI. CHI, fig. le). Swantonia (PI. CIV, figs. 5 and 6) has numerous radiating costse. 



INTERLOCKING PLICATIONS. 



Interlocking of the margins of the valves began as soon as plications or ribs first appeared 



on the shell. The wide variation in size and number in the same species (PI. CHI, figs. 1, la-g) 



indicates that this character is, like that of surface ornamentation, of little more than specific 



value. In the description of the surface ornamentation of the Protremata (p. 302) reference 



is made to the order of appearance and the character of the ridges or plications among the 



Cambrian brachiopods. 



CARDINAL AREA. 



ATBEMATA. 



In the protegulum stage of the atrematous shells the posterior margins of the valve are 

 not bent regularly inward toward each other and there are no indications on the surface of a 

 cardinal area. There is a simple cleft, the length of which is dependent on the degree of round- 

 ing of the posterior margins. Among the genera and species referred to the order Atremata, 

 the pseudodeltidium of Rustella edsoni (PL I, figs, lb, Ic, le) is extremely rudimentary, if devel- 

 oped at all, and there is no trace of a thickened cardinal surface. The pedicle appears to have 

 impressed itself on the shell to the extent of making a shallow furrow on the posterior inner 

 margin of the rather thick shell of the two valves. In MicJcmtzia monilifera (PI. VI, fig. 1") 

 the pressure and movement of the pedicle against the vertical cardinal margin during the 

 growth of the shell have had the eff'ect of bulging it outward so as to produce a pseudodeltid- 

 ium, but on neither of the valves is there a trace of a cardinal thickening. In Helmersenia lado- 

 gensis (PI. LXIII, fig. 7d) a pseudodeltidium similar to that in Mickwitzia occurs on the ventral 

 valve, and here also there is only a thickening of the posterior margin and not a true cardinal 

 area. In the genus Micromitra the mechanical influence of the pressure of the pedicle on the 

 ventral valve is muchmore pronounced; in Micromitra (Paterina) hella (PI. II, figs, la and lb) the 

 pseudodeltidium is more clearly defined than in Mickwitzia or Helmersenia, and the concave 

 arch of the cardinal margin for the protrusion of the pedicle is greater. In Micromitra {Paterina) 

 superha (PI. II, fig. 7) the excavation of the pseudodeltidium has increased, and in Micromitra 

 (Paterina) logani (PI. II, fig. 6b) and Micromitra (Iphidella) pannula (PI. IV, figs. Id, 2a", 

 and 2b') the pseudodeltidium has been excavated to such an extent that there is nearly an 

 open delthj^rium. This difi"erence in delthyrial openings must be due to variation in the thickness 

 of the pedicle. Attention is called to the pseudodeltidium shown in figure 2a", Plate IV, where 

 the tripartite, longitudinal division recalls the pseudodeltidium of Acrotreta nicholsoni (PI. 

 LXXIII, figs. Ig and Ih). So far as known to me, no true cardinal area is present in any species 

 of the Paterinidse, though there may be a more or less marked thickening or bulging of the 

 posterior margin of the valves to form a pseudodeltidium. 



In the OboHdse the vertical cardinal area is present in all known species, but in varying 

 degree of development. It is shghtly shown in Delgadella (PL XXIX, figs. 5, 5a, and 5b) and 

 well defined in Oholus ]}rindlei (PL XXVII, figs. 3a, 3b, 3c, and Be), Lingulella granvillensis 



