348 Dr. H. A. Nicholson on the British 



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 

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

 further and later development of the primitive type of the 

 genus, since they are unrepresented in rocks older than the 

 Upper Llandeilo. The primitive type, however, does not 

 cease to be represented with the Skiddaw and Quebec groups ; 

 for D. March isoni is characteristically Upper-Llandeilo, and 

 D. serratidus 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 allude to the so-called Didymo- 

 (jrapsus caduceus, originally described by Mr. Salter from 

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

 afterwards figured by him from the Skiddaw Slates {ihid. 

 vol. xix. p. 137, figs. 13, a, b). 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 

 SkiddaAv Slates (including Salter's original specimens) has 

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

 examples from this formation. I), caduceus, namely, as de- 

 scribed by Salter, was unquestionably founded upon fragmen- 

 tary examples of the four-stiped Tetragrajysus hryonotdcs, liaWj 

 or of the hardly separable Tetrograjmus ( GraptoJithns) Biyshyij 

 Hall. Recently Mr. Baily has stated that Didymogra2)sus 

 caduceus, Salter, occurs abundantly in strata 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, I 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 apply to forms different from those originally 

 ])laced under it by Mr. Salter. It appears, hoAvever, very un- 

 likely that the genus Tefrayrcqysus, 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 Didymograpsi. 



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

 caduceus^ Salter, has certainly four branches, but still places 

 it under Didymograjysus — a position obviously unsuited for it, 

 whilst he does not recognize its unquestionable identity with 

 Tetragrapsiis hryonoideSy which he also gives as a Didymo- 

 grapsus*. 



* It being now certain tliat tlie specimens originally described by 

 Salter as D. caduceus are really referable to that afterwards named by 



