Clarke — The Naples Fauna. 



123 



emphasizes the orthoceran origin of the genus;* nor does it seriously militate 

 against the cautious statement of Hyatt :f ''It is quite possible that B.u - 

 trites of the Devonian may be a degraded form of Mimoceras, but in that 

 case the latter is also a degraded form of Axarcestes or transitional between 

 it and Bactrites. * * The straight BACTRiTES-like young of some forms 

 of Anarcestes, the gyroceran young of others of the Groniatitinse and the gyro- 

 ceran adults and young of Mimoceras, indicate the derivation of the Goniati- 

 tinae to have been from Silurian straight shells similar to Bactrites, if not 

 directly from that genus itself." 



In the correlation of present evidence relating to the ontogeny of Bac- 

 trites, it will, perhaps, be helpful in explaining apparent discrepancies in the 

 result, to keep in mind the fact that the single protoconch ascribed by Beaitco 

 to Bactrites (species not determined) is from a lower Devonian horizon, 

 while the numerous examples here recorded are from now well known and 

 typical species, B. gracilior and B. adculum, appertaining to a much later 

 (lower upper-Devonian) fauna. 



Specimens of Bactrites are common throughout the Intumescens 

 province of New York, but because of their slender pencil-like cones, good 

 examples of adult shells are very rarely found. The young stages which 

 have been studied, indicate that the two species differed in the size of the pro- 

 toconch and the degree of expansion of the tube, but the outcome of these 

 differences in the adult is not in every respect perfectly clear. The majority of 

 all of these young shells seem to agree among themselves and possess many 

 of the characters which have been ascribed to Saxdberger's species, Bactrites 

 gracilis, a shell which is found in certain facies of the Intumescens fauna, as 

 for example, in the Iberg fauna of the Hartz mountains. To this species the 

 writer was at one time disposed to refer one of the New York forms. With 

 fuller knowledge of this shell it now seems necessary to acknowledge its 

 specific difference. 



* Id Calalogue of Fossil Cephalopoda in the British Museum, Part III, by FooRDand Ckick, 1897, these later results have 

 escaped notice. 



tGenesis of the Arietidae. 



