492 



Forty- fifth Report on the State Museum. 





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■v\S».« 







they are frequently visible to the unaided eye ; in Syringothyris 

 they are often difficult of discernment, and among fossils gener- 

 ally have probably been to some degree obscured by the process 



of fossilization. In smooth shells 

 they are usually equally dissemina- 

 ted, but in plicated species they 

 are arranged in rows on the plica- 

 tions or in the furrows. They 

 may traverse the shell vertically, but 

 their tendency is usually obliquely 

 forward in the direction of shell- 

 growth, and in some instances 

 (Tropidoleptus carinatus) they con- 

 verge toward the plications. Such 

 variations in arrangement may leave 

 fig. 94.- An enlargement of the shell certain portions of the sheH impunc- 



structure in Tropidoleptus; showiDg the . . . 



convergent tubules. tate while other portions are richly 



supplied with tubules. 



The full taxonomic significance of these test-perforations 

 and their function, is not as yet understood. In certain groups 

 of shells some of the members may be punctate, others impunc- 

 tate. Thus among the orthids we find that the earlier and 

 typical members (0. callactis, etc.), are completely impunctate ; 

 so also are Plattstrophia, Hebertella and Dinorthis ; while the 

 greater number of species belonging to the group, especially its 

 later (Upper Silurian, Devonian and Carboniferous) members, are 

 richly punctate. The shell of Spirifee, again, is 

 normally impunctate, while the closely related 

 genera Cyrtin a, SpiRiFERiNAand Syringothyris are 

 punctate, and even a few species which must still 

 vSjik be referred to the genus Spirifer have a sparsely 

 punctated shell (8. plenus). Khynchonella and its 



Fig. 95, 

 largement of an 



l " recent and fossil allies are impunctate, except 



of slayer tfStZ?; Rhynohora and Rhychopoba, which are simply 

 p^c£tion B he sh0,i rhynchonellids with punctate shell. 



There are certain conditions which appear to have induced the 

 deposition of impunctate shell-matter even in species uniformly 



44 



