DIDYMOaRAPTUS. 31) 



('Sil. System,' pi. xxvi, fig. 4) are on a solid block of limestone about 6*3 cm. thick, 

 which bears numerous Graptolites on both sides. The specimens figured in the 

 ' Silurian System ' do not occur in juxtaposition as drawn upon his plate, but are 

 scattered over both sides of the stone, and upon some of its fragments. It seems 

 clear that in Sowerby's figures two fairly distinct forms are included : 



1. A form in which the proximal end is thick, and in which the stipes are usually 

 more or less parallel or convergent ; 



2. A form in which the proximal end is very slender, the increase in width 

 I'apid, and the stipes as a rule either continuously divergent or divergent distally. 



NoAv while it seems certain that both forms were originally included in the V. 

 Murchisoni as figured and described, it also appears evident that the second of the two 

 forms is that which Swedish palaeontologists recognise as the form which Hisinger 

 afterwards described as D. geminus. The two forms occur in association on the 

 same slabs in Britain, and it is possible that they may eventually be shown to be 

 identical; at all events, we can only regard D. geminus as a variety or mutation of 

 D. Murcldsoiii. This was the view taken by Tullberg in 1881 ('Kong. Sven. Vet. 

 Akad. Handl.,' No. 13, p. IG), though the typical form of D. Murchisoni had not 

 l)een recognised by him in Sweden. We hold that at present the balance of 

 evidence is in favour of the varietal character of this second form, and we here 

 restrict the name D. Murchisoni to those forms possessing a thick proximal end 

 and more or less parallel stipes, and we employ the name var. geminus, His., 

 for the second form only. 



The well-known specimens of the D. Murchisoni group which occur in such 

 abundance at Abereiddy Bay would seem at first sight to present certain differences 

 from the typical form, the average number of thecse to the cm. being fewer as a 

 rule. It is, however, certain that the rocks at that locality have been much cleaved, 

 and the fossils consequently much distorted. In correspondence with this we find, 

 on one and the same slab, greatly elongated and narrow forms, Avith distant tlieca?, 

 lying more or less at right angles to forms much shortened and widened, and Avitli 

 close-set thecse, which might naturally be taken for distinct species. But the two 

 extremes are connected by transitional forms lying at intermediate angles, in 

 spite, however, of this deformation we are still able to distinguish the tyj)e 

 D. Murchisoni from its mutation D. geminus, to which latter we would refer 

 Lapworth's T). furcillatus. 



Affinities. — D. Murchisoni (as above restricted) is closely allied to var. geminus 

 and D. hificlus. 



From var. geminus it differs, as has been shown, in the characters of the 

 proximal end; in addition it may be noted that the immber of thecse in a given 

 miit of length is generally higher than in the variety, for while the number 

 to the cm. may vary from twelve to sixteen in the typical form, and is most 

 commonly fourteen, in the variety the number rarely exceeds twelve. 



