CAMBRIAN r.RACHIOPODA WALCOTT 8l 



Surface marked by a few concentric lines of growth. The shell 

 is rather thin except over the umbonal and posterior portions of the 

 ventral valve, where it is moderately thick. Its substance is now cal- 

 careous and appears like that of T. i)isigius; the original shell may 

 have been calcareo-corneous. A ventral valve i8 mm. in length has 

 the same width ; a large dorsal valve 22 mm. long has a width of 

 27 mm. 



The area of the ventral valve is short and divided midway by a 

 depressed subtriangular false pedicle furrow ; the presence of pits 

 on each side of the antero-lateral margins of the false pedicle furrow 

 indicates that the area at these places projected in the same manner 

 as that of T. insigiiis and T. kempamim (Matthew) [1897, p. 70]. 

 The cast of the interior of the ventral valve shows a median ridge 

 with the cast of the pedicle opening at about the posterior sixth of 

 the length of the valve ; only the imperfect outlines of the splanch- 

 nocoele are known ; the main vascular sinuses are outlined for a 

 short distance back of the transverse center sufficiently to indicate 

 that their position was about the same as in T. kempammt. The 

 position of the anterior lateral muscle scar is clearly shown just out- 

 side of the main vascular sinuses ; it is elongate oval in outline and 

 a little in advance of the transmedian scar; the umbonal muscle 

 scars are close to the median furrow, as in T. kempamim. Interior 

 of dorsal valve unknown. 



Observations. — ^This fine species differs from all other known 

 species of the genus in having the ventral valve more convex than 

 the dorsal and in its greater size. It is the oldest species of the 

 genus, occurring as it does well down in the section of the Lower 

 Cambrian strata in association with Olenellus. There are a large 

 number of specimens of the exterior of the dorsal valve, but only 

 two interior casts of the ventral valve. The dorsal valves of T. 

 pristinus (Matthew) [1895, p. 121] and T. kempanum also greatly 

 outnumber the ventral valves in the collections. This circumstance 

 may be owing to the presence of the foraminal furrow and interior 

 median furrows ; these would cause the shell of the ventral valve to 

 break more readily than that of the dorsal. 



Formation and Locality. — Lower Cambrian: (i) Shales and 

 interbedded limestones at the south end of Deep Spring Valley; (2) 

 shales at a higher horizon than (i), but at the same locality; (3) 

 sandstone on ridge east of the head of Mazouka Canyon, Inyo 

 Range; and (4) arenaceous limestone one mile (1.61 km.) cast of 

 Saline Valley road, 2 to j? miles {3.22 to 4.83 km.) east-northeast of 

 Waucoha Springs ; all in Inyo County, California. 



