BV JOHN MITCHELL. 445 



instance, in the present form there is a partial fusion of the hrst, 

 second and third pairs of glabellar lobes by the partial or com- 

 plete obsolescence of the second and third pairs of glabellar fur- 

 rows as they approach the axial grooves ; but this obsolescence has 

 not sufficiently progressed tu affect the pentamerous character of 

 the glabella, it further differs from the genotype by the inter- 

 ruption, centrally, of the hrst pair of glabellar furrows, which, 

 therefore, do not detach the frontal lobe from the rest of tne 

 glabella, as occurs in the case of D. caudatus. In these two modi- 

 hcations it shows the first steps of the specialisation developed so 

 fully in the true L'haeojjs group, but it is remarkable that, in the 

 Australian Palaeozoic seas, as far as research has yet revealed, 

 a group represented by the species now under discussion, and by 

 D. {Hausmannia) meridianus E. and M., exhibiting only a small 

 degree of specialisation, should make its sudden appearance, asso- 

 ciated with such highly specialised forms as Phacops crossleii E. 

 & M., and P. latigenalis E. & M,., and by the very highly specialised 

 form described further on, unaccompanied by intermediate forms. 

 Besides the specialisation of the glabella? noted above, the 

 pygidia of our members of the Dalmanites branch, show tran- 

 sition towards the pygidial segmentation of the Devonian species 

 of the branch, but do not exceed the segmentation allowed to be 

 the limit for Silurian species by F. R. Cowper Reed (Geol. Mag., 

 N.S., Dec. v., Vol. ii., 1905, pp.172-178 and 224-228), viz., 12-16. 

 In the present species the pleural segments of the pygidium in 

 individuals nearing maturity range from twelve to fourteen, and 

 in D. (Hau&mannia) meridianus ten to twelve. The largest 

 pygidium of the species now described possesses twenty axial rings 

 and fourteen pairs of pleural segments. In D. meridianus these 

 similar divisions are seventeen and twelve respectively for the 

 largest known specimens. Mr. Reed (loc. cit.) refers to the 

 absence of the Silurian forms of Dalmanites from Bohemia, and 

 particularly from the typical Silurian horizon, Barrande's etage E. 

 It is somewhat interesting to note that the lower Trilobite Beds 

 of our Bowning Series, judging by the trilobite fauna they have 

 yielded, are homot axial with the beds forming Barrande's etage E. 



35 



