Shimer — Specimen of Sirenuella strenua (Billings). 201 



of the bead shield, in the long eye-lobes, the depressed anterior 

 limb of the cheeks and in the short and direct posterior exten- 

 sion of the facial suture. It is also noted that it resembles 

 Liostracus in the prominent glabella with the depressed area 

 behind the anterior margin of the head-shield. In all of these 

 points, the specimen discussed here agrees with the generic 

 description of Strenuella ; it is apparently the first of the 

 species strenua to be described possessing a thorax. 



That this specimen does not belong to Walcott's varietal 

 form of S. strenua nasutics, is indicated by the following 

 measurements. This variety was founded on a specimen* 

 which has " a broad, extended frontal limb." These measure- 

 ments used for comparison were taken from cuts. The rela- 

 tive widths of the cephalon, glabella, etc., are alike in the 

 species and variety, hence the comparison is made on the length 

 alone. 





From 



anterior 



Width of 







edge of 



glabella to 



frontal 





anterior neck furrow 



limb 6 



Proportion 7 



S. strenica 1 





7 



3 



1 : -43 



a. a 2 





7 



2 



1 : -29 — 



a a s 





10 



2-75 



1: -27 + 



a a * 





5'5 



2 



1 : -36 — 



v&r. nasutus* 





5-5 



3 



1 : -55 — 



The measurements are in millimeters. 



1 Figure of the type specimen. — Billings, Geol. Surv. Can., 

 Pal. fossils 2 ; 1, pp. 71, 72. 



2 Walcott, U. S. G. S. Ann. Rep. 10, pi. 97, fig. lb. This is 

 said by Walcott to be much like the type of the species. 



3 Grabau, Occas. Papers, Bos. Soc. Nat. Hist., iv, pi. 34, fig. 7b. 



4 The specimen here discussed. 



6 Walcott, loc. cit., fig. 2. Type specimen. 



6 Distance from the glabella to the anterior edge of the cepha- 

 lon. 



7 Letting length of glabella from its anterior edge to the ante- 

 rior neck furrow = 1. 



It thus appears that S. strenica has a frontal limb which in 

 all the specimens examined, bears to the length of the glabella 

 from its anterior edge to the anterior neck furrow, the relation 

 of less than one-half to one ; while that of the variety nasutus 

 is more than one-half to one. The present specimen then 

 agrees much more closely with the type of 8. strenua than it 

 does with the varietal form nasutus. 



Geological Department of Massachusetts, 

 Institute of Technology. 



*U. S. G. S. Ann. Eep., 10, 654, pi. 97, fig. 2, 2a-c. 



