1865.] 



Notes and Correspondence. 



363 



ones and that in question, occur in 

 Cretaceous rocks. The' genus is 

 pre-eminently Jurassic, but extends 

 upwards into the chalk. Everyone 

 knows that the Jurassic Mammals 

 are very nearly related to the Mar- 

 supials of Australia, and tbat this 

 curious relation has been used by 

 geological theorists, much as the 

 unfortunate frog has been tortured 

 by experimenting physiologists. It 

 is also well known that the Jurassic 

 facies of the Australian terrestrial 

 fauna is strengthened by the exist- 

 ence of three or four species or 

 varieties of Trigonia in the neigh- 

 bouring seas. But matters seem 

 to have reached a climax, now that 

 in the Tertiary beds of the same 

 great island there has been found a 

 Trigonia which is much more nearly 

 related to the Oolitic than to the 

 recent species, for it belongs to the 

 same type as the former, and to a 

 different one than the latter. 



The recent species have radiating 

 ribs commencing at the umbo, and 

 proceeding straight to the margin ; 

 they belong to the ' ' pectinate " 

 division of the genus, and indeed 

 entirely constitute it. The Tertiary 

 species, which has received the 

 name of T. subundulata from Pro- 

 fessor M'Coy, though it has never 

 been described, belongs to the 

 "costate" group, which is mainly 

 represented in Jurassic rocks, but 

 a few species of which pass up into 

 Cretaceous strata. The chief cha- 

 racteristic of this group is that the 

 anterior portion of the shell is or- 

 namented by longitudinal ribs, 

 which stop short where they meet 

 the keel, extending from the umbo 

 to the lowermost of the three pos- 

 terior angles, from which the genus 

 derives its name ; behind this the 

 ribs are nearly vertical, that is to 

 say, parallel to the keel, and nearly 

 at right angles to the longitudinal 

 ribs already mentioned. In this 

 chief feature Trigonia subundulata 

 resembles all the species of the 

 " costate " section, coming nearest, 

 perhaps, to Trigonia suprajurensis, 



VOL. II. 



Agass.; but it should be mentioned 

 that the keel is not so prominent 

 as in other " costate " species, being 

 scarcely more so than the rest of 

 the ribs. 



Every palaeontologist believes 

 that, when a genus of animals is 

 represented by species occurring in 

 strata of widely different ages, it 

 must have been perpetuated by 

 some one or more species during 

 the whole of the intervening period. 

 In fact, it is generally assumed that 

 a genus or a species never reap- 

 pears after it has once died out, 

 and palaeontologists feel as certain 

 of the truth of this postulate as of 

 that of the first axiom in Euclid. 

 The same law would generally be 

 allowed to hold as good for families, 

 orders, and classes, as for speeies 

 and genera. 



It seems paradoxical that the 

 most ardent admirers of the " final 

 cause" line of argument, as op- 

 posed to the empirical, should be 

 the most strongly opposed to an 

 application of their practice to this 

 curious subject ; but such appears 

 to be the case. The reason is ob- 

 vious. The only rational meaning 

 that has ever been attached to this 

 presumed general law (for it is in- 

 capable of proof in many cases) is, 

 that the perpetuation of the genus, 

 species, family, &c, as the case may 

 be, has been due to " descent with 

 modification." The accident of the 

 intermediate links being unknown 

 in such cases, when everyone be- 

 lieves them to have existed, supplies 

 an excellent parallel to the frequent 

 absence of the fossil remains of the 

 " connecting links " between groups 

 of animals, especially as this absence 

 has so frequently been a stumbling- 

 block in the path of the students of 

 Mr. Darwin's theory. The absence 

 in the one case appears to be due to 

 the same causes as in the other ; 

 and if the want of evidence be not in- 

 compatible with former existence in 

 the former, neither is it in the latter. 



Trigonia subundulata is one of the 

 links hitherto wanting ; first, in ex- 

 2c 



