184 DR. r. J. NOETH OS [vol. lxxvi, 



Z, Malahide ; C, Adare (Limerick) and Rathfeale (Cork) ; and 

 from the Dibunopliyllum Zone in the Midlands and Yorkshire (as, 

 for example, Coniston Cold, Skipton l D„, Kniveton). 



As in the case of S. cuspidata, this species attains its maxi- 

 mum size in the Waulsortian 'knoll '-limestones of Ireland. 



Sykingotityeis cuspid at a (Martin). 



In this species are included shells from Z to D inclusive, in 

 which the brachial valve is about twice as wide as high, apical 

 angle about 90° to 100° (100° in the earlier, and 90° in the later 

 mutations), cardinal extremities rounded. Mutations characteristic 

 of various horizons are recognized, together with variations due to 

 geographical distribution and to environmental conditions. 



Syrhstgothykis cuspidata mut. CYETORHYXCHA. (Text-fig. 3d> 

 p. 178 ; PL XI, figs. 3 a & 3 h.) 



1825. Spirifer cnspidatus (Martin) — Sowerby [32] p. 90 & pi. cccclxi, figs. 1-2. 

 1911. Syrhiguthi/ris cf. te.vta Hall — Vaughan, ' Burrington Paper' [28 u p. 365. 



Description: — Pedicle- valve. — Area flattened near the 

 hinge-line, but concave towards the beak, which is pointed ; apical 

 angle about 100° : sinus wide, evenly concave from side to side, and 

 convex from the beak to the anterior margin. Costae. about twenty 

 on each lateral slope, low and rounded, and separated by narrow 

 furrows ; lateral slopes tumid, and making an obtuse angle with the 

 cardinal area. Delthyrial supporting-plates stout, divergent, but 

 less so than in S. principalis ; syrinx relatively large, not deeply 

 buried. 



Brachial valve evenly convex; fold distinct and rounded; 

 cardinal extremities almost rio-ht-anodes ; beak small, and but little 

 raised above the level of the area. 



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

 along hinge-line, 40 mm. ; length of brachial valve, 22 mm. ; 

 height of area, 17 mm. 



Owing to the lithological characters of the rocks (of the Za- 

 plirentis Zone) in which this mutation occurs in the South- 

 western Province, the surface of well-preserved specimens shows 

 the characteristic minute ornament, resembling twilled cloth. 



Example. — A specimen from the Avon Section, Bristol (Z), in 

 my collection, and now deposited in the National Museum of 

 Wales (Keg. No. 19246 G 2). (Figured in PI. XI, figs. 3 a 

 &3b.) 



Horizons and localities. — The Zaplirentis Zone in the 

 South- Western Province and in Ireland. In the lowest beds of 

 the zone some of the shells are more or less intermediate between 

 this mutation of S. cuspidata and $. principalis, and towards 

 the top of the zone the mutation gives place to 8. cuspidata mut. 

 exoleta. 



1 Specimen in the British Museum (Natural History) Reg. No. B 20247. 



