348 l)i-. II. A. Nicholson on tJu liritish 



may be regarded as no more than an apparent exception to 

 this statement, as the stipes become ultimately horizontal. 

 We may, therefore, conclude, as far as our present materials 

 ffo, that the second and third sections of Didymograpsi are a 

 lurther and later development of the primitive type of the 

 genus, since they are unrepresented in rocks older than tlic 

 Upper Llandeilo. The primitive tyi)e, however, does not 

 cease to be represented Avith the Hkiddaw and Quebec groups; 

 for D. Murchisoni is characteristically Upper-Llandeilo, and 

 D. serratuJus occurs in the Utica Slate (Caradoc) of America. 

 There is, further, one form which would invalidate this gene- 

 ralization, if it were to be established in the position originally 

 assigned to it by its author. I alhide to the so-called Dich/nio- 

 (]raj)sus caduceiiSy originally described by i\Ir. Salter from 

 Canadian specimens (Quart. Joum. Geol. Soc. vol. ix.), and 

 afterwards ligiu-ed by him from the Skiddaw Slates (ibuL 

 vol. xix. p. 137, figs. 13, rt, h). As I have elsewhere stated, 

 there cannot be any hesitation in rejecting, with Hall, this 

 species, as far as the Quebec group is concerned ; and an exa- 

 mination of a very extensive suite of specimens from the 

 Skiddaw Slates (including Salter's original specimens) has 

 fully satisfied me that Hall's explanation applies also to the 

 examples from this formation. D. caduceus, namely, as de- 

 scribed by Salter, was unquestionably founded upon fragmen- 

 taiy examples of the four-stiped Tetragrapsus hrgonoidcs, llnW, 

 or of the hardly separable Tetragraj)sus {GraptoUthus) Bigsbyt\ 

 Hall. Recently Mr. Baily has stated that Didymograjysvs 

 caducens, Salter, occurs abundantly in sti-ata of Caradoc age 

 in Wexford (Quart. Journ. Geol. Soc. vol. xxv. p. 160). Not 

 having had the opportunity of seeing the specimens in ques- 

 tion, 1 do not presume to express any opinion with regard to 

 them, except that, if the name of D. caduceus is to be retained, 

 it must be made to a])]jly to forms different from those originally 

 placed under it Ijy Mr. Salter. It appears, however, very un- 

 likely that the genus Tctragrajysus, which has hitherto not 

 been discovered in any Upper Llandeilo deposit, should have 

 survived into the Caradoc period ; and Mr. Baily's specimens 

 are therefore likely to be genuine Didgrnograjm. 



Mr. Cannithers (Geol. Mag. vol, v. p. 129) admits that D. 

 caduceuft, Salter, has certainly four branches, but still places 

 it under Didgmograpsus — a position obviously unsuited for it, 

 Avhilst he does not recognize its unquestionable identity with 

 Tctragrnpsus brgoiioides, which he also gives as a Didymo- 

 grcqysus*. 



• It being now certain that the specimens oripinally described bv 

 Salter as D. caduceus are reallv referable to that afterwards named bv 



