NO. 7 SILICIFIED ORDOVICIAN BRACHIOPODS ROSS AND DUTRO 9 



at anterior margin of specimen that is 13 mm long; shell impunctate. 

 Pedicle valve apparently with open delthyrium; remnants of small 

 deltidial plates in a few specimens ; small hinge teeth unsupported by 

 dental plates ; umbo extended, not incurved. Brachial valve with faint 

 sulcus ; interior with divided hinge plate ; no evidence of cardinal proc- 

 ess ; strong median septum extends three-quarters the length of valve ; 

 spire with descending lamella in plane of commissure. 

 Measurements in mm: 





Length 



Width 



Thickness 



USNM 145331 

 USNM 145329 

 USNM 145327 (holotype) 

 USNM 145330 

 USNM 145328 



12 



9 



13 



10 



6 



11 



10 



12 



9 



3.5 



4 



3+ (broken) 



8.5 



2 



Discussion. — This species is similar to C. cartieri Cooper and 

 Kindle in the spacing of radial ornamentation but has only about half 

 as many costellae. The outline of C. cartieri is more nearly circular 

 and its pedicle umbo is smaller. C. homeospiroides possesses a faint 

 sulcus on the brachial valve, not on the pedicle valve as in C. cartieri. 



This species is similar to C. arcana Williams from Girvan in size 

 and general shape. However, the brachial valve is not as broadly or 

 deeply sulcate and costellation is a little coarser ; in C. arcana there are 

 about 8 costellae in 5 mm at a distance of 13 mm from the pedicle 

 umbo. In addition, the pedicle valve of the Scottish species is more 

 convex than the brachial, whereas the Alaskan shells are equally 

 biconvex. Catazyga headi is more finely ribbed and more rotund, with 

 a widely V-shaped brachial sulcus. 



Williams (1962, p. 247) states that adult shells tend to be longer 

 relative to width than juveniles in C. arcana, but the opposite seems to 

 be true in the present form, as shown in plate 1, figure 15. 



Although the ribbing is coarser than is customary in Catazyga, and 

 the brachial valve lacks a pronounced sulcus, this species is placed in 

 that genus. Future investigation may show that several impunctate 

 species now classified within Homeospira should be grouped with 

 Catazyga homeospiroides as a distinct genus. 



Approximately ten silicified specimens and two calcareous speci- 

 mens of C. homeospiroides are the basis for the above description. 



Holotype.— USNM 145327. 



Figured paratypes.— USNM 145328, 145329, 145330, 145331. 



