80 



Report of the State Geologist. 



Manticoceras Pattersoni itself prevails and abounds in the more easterly 

 extension of the Intumescens-f auna, in the Ontario and Livingston county region; 

 but west from the Genesee river it becomes constantly more infrequent and in 

 the vicinity of lake Erie its place in the fauna seems to be taken by Mantie. 

 rliynchostoma, the remarkable species which in its mature condition or in its 

 average adult size is wholly indistinguishable from Mantie. Pattersoni, while 

 these ephebic characters are attained in passing through growth stages dis- 

 tinctly distinguishable from those of that species. 



In this westward extension of the fauna occurs also the accelerated species 

 Mantie sororium, whose mode of acceleration is similar to, but more extreme 

 than that of Jf. Pattersoni, var. styliophilum of the prenuncial fauna of the 

 eastward region. Rapid acceleration is especially noteworthy in the species 

 from the higher strata of the formation, especially in the carinate shells 

 Mantie. vagans and Mantie oocy; even, indeed, in Mantie Pattersoni itself in 

 its latest manifestations. 



Manticoceras Pattersoni. 



In assuming Mantie. Pattersoni as the yard-stick with which to measure 

 the various manifestations of the genus here presented, we do this, as pre- 

 viously explained, because it is the most characteristic Manticoceras of the 

 fauna, the most abundant, and the most typical expression of this generic 

 combination. Other species are, hence, most intelligibly expressed in terms 

 of this. It is not regarded as an elementary, so much as a standard 

 expression ; for it exhibits acceleration in several respects. With reference to 

 the simplest expression of the genus deducible from the data, though this 

 expression be not represented by any known species, it is clear that Mantie. 

 Pattersoni is considerably accelerated in the mode of introduction of the 

 ornament, and distinctly if slightly tachygenic in conch-section, umbilication 

 and septation. 



Manticoceras apprimatum. 



The w horl section indicates slight acceleration. The umbilication, w hich 

 is very close, we interpret as indicative of a moderately arrested development. 

 There is no phase of Mantie. Pattersoni w here the umbilication is so slight as 

 here, but this retention of centripety is, in the standard, a condition prevailing 

 in early ephebic stages. In the matter of ornamentation, the simple varices 

 witli undivided interspaces continuing through three Full volutions, show that 

 here is a more primitive expression than in the standard, exhibiting a 

 prolongation of what was there a brief growth period. 



