Report of the State Geologist, 



17? 



The concentric lines when not wholly superficial may be 

 regarded as representing periodic interruptions in the out- 

 ward expansion of the mantle. In some genera, as Obbwuloid] a. 

 Disoinisoa, Tkimikki.i.a. etc., there are fre 

 quent irregular Lamellose expansions of the 

 outer shell layer, which produce a rough, 

 squamous exterior (Plates 5, 6.). In 

 species of Athybis, Atrypa, and. rarely, 

 Peoductus, there are regular expansions 

 at the concentric lines, which are some- 



, . , , . , YlQ.44.—Productu9tesselatus, 



times extravagantly extended (A thuris viewed from the brachial vaive-. 



* „ showing the curved fringes. 



coneentnca, A. planosidcata, A. la/mellosa^ (Davidson.) 



Atnjpa reticularis, etc.): in other species the concentric lamellae 

 become divided into a fringe of fiat, hollow spines. Again, in 



SlPHONOTRETA, AtHYRIS, 



and Spirifer. there are 

 rows of hollow, round 

 spines, which are some- 

 times divided by a 

 median partition (A. 

 hirsuta) and may be 

 compound, with pinnate 

 lateral branches. (S. 

 jimbriatus.) (See Plate 



fig. 



12). 



When 



the spines are irreg- 

 ularly scattered over 

 the surface they are 



generally Of large rFiG 45.— Athyris pUinosulcata, with its marginal expan- 



size. This is preemi- sions - <P"id*wo 



nently the case in Productcs, where the spines are often of great 

 size and remarkable length. Such spines could never have been 

 flexible, but they were frequently a means of attachment to 

 foreign bodies, whether by cementation as in Stkophalosia, or by 

 anchoring in the sediment.* In the younger conditions of such 

 spiniferous shells the spines have, to some extent, opened into the 

 inner cavity of the shell, but in later growth they became, for the 



* It has been stated by Young that in some species of Prodlctus the large spines appear to 

 be furnished on the interior with a multitnde of spinules standing convergent to the axis of the 

 spine. This observation has not yet been verified by others. 



29 



