part 2] SYRINGOTHYjRIS AND SPJBIFEBrNA. 185 



Remarks. — This mutation of S. cuspidata is readily dis- 

 tinguished from S. principalis by its curved area. It is smaller 

 than &'. cuspidata mut. exoleta, but has all the essential characters 

 of that mutation developed in a minor degree. 



Syringothykis cuspidata mut. exoleta. (Text-figs. 1 d, Se & 

 3f, pp. 166, 178 ; PL XI, figs. 4 a-5 b.) 



1858. Spirifera. cuspidata Martin sp. — Davidson [4] p. 44 & pi. viii, figs. 19-20. 

 1917. Syringothyris cf. cuspidata (Martin) — Dixey & Sibly [6] p. 142. 



In general characters, examples of this mutation from normal 

 limestone-deposits are intermediate between the mutation cyrto- 

 rhyncha already described and the typical mutation cuspidata. 



In S. cuspidata mut. exoleta the genus Syrinyotliyris reached, 

 in the British-Belgian area, its acme in the size and number of 

 individuals and in geographical distribution. 



Description (of the normal type characteristic of the South- 

 western Province): — Pedicle-va lve.— Area concave, especially 

 towards the beak; apical angle about 100° ; sinus wide and shallow, 

 the median line along its floor being evenly and markedly convex. 

 Cost* well defined anteriorly, rounded, but depressed, and separated 

 by narrow furrows ; lateral slopes rounded and tumid. Delthyrial 

 supporting-plates slightly divergent ; syrinx well developed and 

 relatively large. 



Brachial valve. — A little more than twice as wide as long, 

 convex, fold well defined, low, and rounded ; beak slightly inflated 

 and raised above the level of the area, but less so than in the later 

 forms. 



Typical examples. — Specimens from Weston-super-Mare and 

 from Sutton, Glamorgan (see PL XI, figs. 4 a & 4 5), in the col- 

 lection of Principal T. F. Sibly. 



Horizons and localities.- — In the South-Western Province 

 this mutation ranges from Z„ to C, and has also been recorded as 

 occurring rarely in S r It does not every where attain its maximum 

 at the same horizon, but reaches its acme in the Bristol district in 

 Z.„ in East Glamorgan at horizon y, and in the Mendip area in C a . 

 Normal examples of this mutation occur in the Caninia Zone in 

 Ireland. 



Variations of mut. exoleta. 



It has already been stated that this mutation is typically 

 developed in the South -Western Province. In the North- West of 

 England it is represented by forms which have all the essential 

 characters of the mutation, but are somewhat less tumid, and 

 in which the flattening of the ribs is more apparent and the 

 beak of the pedicle-valve sharper. This type has been recorded by 

 Prof. E. J. Garwood in the Brownber Pebble-Beds (sub-zone of 

 Sem inula grey art a, lower part of C.,), 1 and in a hard calcareous 



1 Garwood [13] p. 464. 



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