362 



CAMBEIAN BRACHIOPODA. 



deltidium; the latter projects directly outward at right angles to the false area and then curves 

 abruptly, so as to be almost flat across the center. A narrow, slight median groove extends 

 from beneath the apex to the posterior margin. Another specimen shows a portion of a pseudo- 

 del tidium that is somewhat less elevated than the one described. The apex of the valve just 

 outside of the extreme point of the beak is crossed by a very minute longitudinal depression 

 that is visible only under a strong magnifier. 



Dorsal valve slightly convex, sloping regularly from the front margin to the small beak, 

 which is slightly incurved at the margin of the valve. False area clearly defined; in a speci- 

 men 10 mm. in width the area has a width of a little over 1 mm. at the side, narrowing to a 

 point at the apex. It is broken midway by a wide triangular opening, which is filled in by a 

 depressed pseudodeltidium. The sides of the deltidium turn in at nearly a right angle for a 

 short distance to the general plane of the pseudodeltidium, which extends across from side to 

 side. Posterior margin slightly arched, and its general surface broken midway by a narrow, 

 distinct groove, which extends from beneath the beak back to the posterior margin. (See PI. 



IV, fig. If.) Strise of growth extend across 

 the false areas and pseudodeltidia in both 

 valves; shell substance corneous. 



The surface ornamentation of this species 

 is as higlaly ornamental as that of any Cambrian 

 brachiopod. It appears to be formed of a 

 very fine network of oblique raised lines, which 

 divide it up into minute diamond-shaped pore- 

 like pits, a surface which resembles, under a 

 strong lens, the texture of finely woven cloth. 

 A closer examination, however, of some of the 

 larger shells shows, on the outer margin, cren- 

 ulated concentric lines, and a little farther back 

 on the shell more deeply crenulated lines; still 

 farther back the points of the crenulations 

 unite so as to form a solid network that gives 

 the appearance of oblique lines crossing at 

 nearly right angles (PI. IV, fig. Is). 



The surface marking is so strongly char- 

 acteristic that the presence of the species has 

 been detected a number of times by finding 

 very small fragments of the shell. It not in- 

 frequently happens that in old shells the sur- 

 face characters have been almost entirely worn 

 away, traces of the ornamentation remaining 

 The wearing of the surface near the umbo is shown 



FiGTiKE 32. — MicTomiIra (Iphidella) pannula (White). Ventral valve, 

 showing setEe, from Locality 35k, Middle Cambrian, Burgess shale 

 member of the Stephen formation, near Field, British Columbia. 

 This specimen is unique in being the first Cambrian brachiopod in 

 which the setse have been observed. The preservation of so frail a 

 portion of the anatomy of the brachiopod is no more remarkable, 

 however, than the presence in the same shales of aimelids showing 

 the fringe of setge around the mouth; of crustaceans of many kinds 

 showing the branchife and appendages; of trilobites with antermse, 

 branchiue, legs, etc.; of holothurians showing the podia and the 

 dental plates; of medusEE, sponges, etc.— a fauna which is now being 

 described and figured in volume 57 of the Smithsonian Miscel- 

 laneous Collections. 



only in the vicinity of the cardinal slopes, 

 in Plate IV, figure Im. 



In some specimens from Montana (Pi. IV, fig. Is) the surface is most beautifully pre- 

 served, and at first glance there appears to be a specific difference between it and the specimens 

 from Mount Stephen, central Nevada, and the Colorado Canyon. This may be seen by com- 

 paring figures Ir and Is (PI. IV). There are, however, intermediate forms that possess more 

 or less of the characteristics shown by the two figures. In some specimens there are fine, 

 radiating undulations extending from the apex to the margin that give to the shell an appear- 

 ance much like that of the young of M. sculptilis (Meek) and M. (Paterina) stissingensis (Dwight). 

 These are finely shown by Plate IV, figure In. The surface ornamentation is much like that 

 of M. (Iphidella) ornatella (Linnarsson) of Sweden. It differs but slightly in the form of the 

 depression formed by the \uaion of the crenulated strise. 



This species has a wide geographic distribution, and it also occurs both in the Lower Cam- 

 brian in the Olenellus fauna, and in the Middle Cambrian in the Olenoides fauna. Its range in 



