Report of tee State Geologist. 



491 



Among the articulate genera, under favorable preservation, 

 there may be distinguished three distinct calcareous shell layers; 

 an inner prismatic or fibrous layer, which 

 constitutes the greater portion of the 

 shell; above this is a thin Lamellar layer, 

 and the outer surface of the shell is cov- 

 ered by a tenuous epidermal film or peri- 

 ostracum. When the shell is punctate, 

 as is frequently the case (the orthoids, JSJi'ZS^^^^ 



CyETINA, SpIKIKERINA. the prodllCtoids, °- Piratic layer; m outer, fan 



' J ' punctate layer; n, intermediate, 



terebratllloids) the tubules Open On the laminar layer; /, broad extremity 



J L of perforation; eradiating grooves 



inner surface in narrow apertures, whence about extremity.— (King > 

 they widen upwards, abruptly expanding in the lamellar layer, 

 at whose upper margin they terminate. They do not pierce the 



*..j 



m- 



■ •■-" 



Fig. 92.— Inner surface of a 

 valve of Productus semireticu- 

 latus, showing the opening of a 

 spine (s) and the elevated ter- 

 minations of the punctae (o). 

 (Davidson.) 



Fig. 93.— Portion of outer surface 

 of Productus longispinus. s, spine; 

 c, imperforate outer layer; 6, punc- 

 tee of inner Jayer. (Davidson.) 



periostracum, and it has not been demonstrated that any brachio- 

 pod shell is completely traversed by them. Certain species of 

 Orthis (Rhipidomella) have been shown by Young to possess 

 epithelial punctures which were probably the points of insertion 

 of short spinules ; these coexist with fine tubules whicn perfo- 

 rate the inner layers. The coarser spines occurring in such 

 genera as Sjphoxotreta and Productus are hollow, and may 

 sometimes penetrate the entire substance of the shell. The per- 

 forations vary greatly in size and number. In the terebratuloids 



43 



