200 DR. F. J NORTH ON [vol. lxxvi. 



(b) Ttlothtris laminosa mut. y. Mutation y is the form 

 which most closely approximates to the description of the species 

 already given. • 



Horizons and localities. — Mutation y is particularly 

 characteristic of the upper beds of the Lower Avonian in the 

 South- Western Province. It does not everywhere attain its maxi- 

 mum at the same horizon. In the Mendip area it is particularly 

 characteristic of Z.,, of Horizon y in the east of Glamorgan, and 

 of the laminosa dolomites (C) in the Avon Section. 



The dimensions of an average specimen are as follows : — width 

 along the hinge-line, 30 mm. ; length of brachial valve, 15 mm. ; 

 height of area, 8 mm. Generally speaking, there is a slight increase 

 in size and a greater development of the apical callosity in the shells 

 from Horizon y and C v but the differences are small ; and, owing 

 to the varying mode of preservation, the form characteristic of 

 any particular horizon cannot be definitely described in exact terms. 



Further notes on T. laminosa. — Locally, the species 

 reaches larger dimensions, and in such places usually seems to be 

 particularly abundant. This is noticeable, for example, at Bany 

 (Glamorgan), where in size, and in the intensification of certain 

 characters the shells from y— C recall the special forms of Syringp- 

 thyris found in the knoll-limestones, and in the Midland 'Brachio- 

 pod-Beds.' An unusually large specimen from the Cadoxton inlier, 

 near Barry, had the following dimensions : — width along hinge-line, 

 40 mm. ; height of area, 12 mm. ; length of brachial valve, 20 mm. 1 

 T. laminosa is not widely distributed above C r Vaughan mentions 

 it as being rare in C 2 in the Gower Peninsrda, 3 and as occurring in 

 abundance in a band at one level in S, in the Avon Section. 3 



In Ireland it has been found in Z (? C,) at Hook Head and at 

 Malahide ; in Z, (but not in CJ in Clare 4 and at Bundoran. 

 Large and small forms occur, as in England and Wales ; but there 

 is not at present sufficient evidence to admit of a discussion of the 

 zonal significance of the species in all the localities mentioned. 

 The typical form occurs also in Belgium, as, for example, at 

 Allain. 5 



The cardinal extremities are frecpiently broken off, as in the 

 specimens from Burrington figured by Vaughan 6 : this gives to the 

 shell a deceptive rounded appearance, but the precise nature of the 

 cardinal extremities is usually indicated by the imbricating growth- 

 lines, when they are preserved. The loss of the cardinal extremities 



1 In this connexion it is interesting to recall Mr. E. E. L. Dixon"s suggestion 

 that knoll-reefs may be expected to occur in the Barry area, similar to 

 those discovered near the junction between the Lower Avonian and the Upper 

 Avonian in Pembrokeshire. In the discussion of a paper by P. Dixev & 

 T. F. Sibly [6] p. 162. 



' ? Vaughan [7] p. 546. 3 Vaughan [33] p. 195. 



4 Douglas [8] p. 549. 



3 Vaughan [35] p. 39 ; see also pi. vi, figs. 1-3. 



Vaughan [28] pi. xxxi, fig. 8. 



