McKay. — On the Genus Ehynchonella. 897 



Fossils belonging to this genus abound in the cretaceous rocks of England, 

 and most other countries in the Northern Hemisphere. About twenty spe- 

 cies have been described from the English deposits of this age. The con- 

 trasting scarcity of EhynchonellidaB in the New Zealand cretaceous rocks 

 appears something like evidence in favour of the opinion, that the decline of 

 the genus commenced earlier in the southern than in the northern hemis- 

 phere. Whether or not this speculation has the importance which I would 

 thus attach to it, curiously enough, it finds partial confirmation in the fact 

 that the molluscous fauna of our cretaceous seas assumed a tertiary type of 

 character considerably prior to the period of such change in the Northern 

 Hemisphere, within the European area. 



In the middle and lower- secondary rocks of this country BhyncJionella is 

 represented by a great increase of the number of species, and in many 

 strata the rocks are crowded with such shells. With this increase in 

 number there appears to be a restriction of the vertical range of the dif- 

 ferent species, and a consequent increase of their value for determining 

 different geological horizons. Bhynchojiella nigricuns can only tell us that 

 we are dealing with tertiary strata, and its presence indicates no particular 

 division or group of strata — as eocene, miocene, or pliocene ; and R. 

 squamosa does not do more for our cretaceous strata ; but in our oolitic, 

 liassic, triassic, and permian rocks, the case is different, and each par- 

 ticular fossiliferous horizon may generally be determined by the presence of 

 a peculiar species of Rhy7ichonella. 



Hence the increased importance of the genus in these formations, and 

 the necessity for studying those characters by which it may be identified. 

 Some of the species from these older rocks appear as if considerably dif- 

 ferent from the ordinary types of the genus Ehychonella in outline and 

 other external characters, and one or two species are readily mistaken for 

 Terebratulidae, more especially if the specimens are found as casts, thereby 

 loosing the means for determining such forms by the impunctate structiu:e 

 of the shell. 



Specimens thus liable to be mistaken are abundant in the triassic strata 

 of the Kaihiku Eange, Otago ; and not until the discovery of testiferous 

 specimens was it apparent that these did not belong to the genus Terehratula, 

 which they closely resemble. 



On making the discovery that the shell- structure of this species was im- 

 punctate, I at the same time observed that the hinge-teeth were minutely 

 crenulated, or more properly denticulate, a character not seen in Terehratula. 

 Further examinations showed that specimens, which without doubt belonged 

 to Ehynchonella, had teeth marked in the same manner. 



This discovery led to an examination of other and distinct species 

 of Ehynchonella, in which this character was also seen. 



