200 INFERIOR OOLITE AMMONITES. 



The Hollow-Caeinatb Orammocerata. 



The species which we have now to discuss are easily separable from the other 

 members of the genus Grammoceras by the fact that they possess a hollow carina, 

 which is generally very prominent. Two of the species, Gramm. dispansum and 

 Gramm. metallarium, are so easily recognised by their tubercles from the other 

 members of this group that we may pass them at once ; another species, Gramm. 

 nannodes, is very distinct on account of its combination of dwarf size and adult 

 characters : but the remainder — the Fallaciosum-gromp — require some introduction. 

 They are Gramm. quadratum, subquadratum, Scemanni, Muelleri, fallaciosum, 

 Gotteswoldice, which have generally been referred to by the name Am. radians (see 

 page 188). 



The relationship of these species one to another is undoubtedly very close ; and 

 it is oftentimes a hard matter to decide how such a group may be most 

 conveniently divided. The ribbing remains practically the same throughout, 

 and the only differences among the forms are in shape of whorl and amount 

 of involution. In this group Denckmann has made two more species, Am. 

 Struckmanni and Am. Bingmanni ; but I have preferred to consider these forms 

 as varieties of Gramm. fallaciosum, because, although it is easy enough to see the 

 differences between the extreme forms, yet it appears to me almost hopeless to 

 separate a large series of specimens with any satisfaction. Gramm. Cottesivoldice, 

 again, is little better than a variety of Gramm. fallaciosum, — it is simply the more 

 involute development thereof; but it stands out rather more conspicuously on 

 account of its small umbilicus and compressed whorls. 



Between Gramm. quadratum, siibquadraPmn, and Scemanni there does not seem 

 to be any very sharp lines of distinction ; yet the forms are sufficiently separate for 

 our purpose, and may be divided with very slight attention. 



Among these species the normal process of development appears to be trace- 

 able, namely, from evolute quadrate whorls {Gramm. quadratum) to involute 

 compressed whorls {Gramm. Gotteswoldice) ; and, looking at this fact, it seems 

 very singular that they should all occur on the same horizon — the Jurense-zone 

 (p. 208). However, from Gramm. quadratum to subquadratum, Scemanni, 

 Bingmanni, fallaciosum, Cottesivoldice, appears a perfect " ascending series," 

 involving not only the gradual changes of whorl shape, but also a corresponding 

 increase in the fineness of the ribbing. 



Gramm. Muelleri seems to be a mutation of Gramm. subquadratum. It has 

 attained to greater involution without much loss of thickness. Gramm. 

 Struclcmanni I take to be an aberrant form of Gramm. Bingmanni — a form which 

 had gone back to a wider umbilicus and yet had decreased in thickness. 



