182 BE. F. .T. NORTH ON [vol. lxxvi, 



simple forms is represented by the earlier mutations of both 

 cmpidata and elongata : the adult stage, in which the full size is 

 attained and all the characters are mature, is seen in the mut. 

 exoleta and in the G«iiini<f-Zoi\e representatives of 8. elonqata ; 

 while phylogenetic old age with 'exaggeration and per-elaboration,' 

 followed quickly by extinction, 1 was reached (in normal limestone- 

 deposits ) in the TJibunopli.yllnm Zone, but was, in certain areas, 

 hastened by the special conditions accompanying the formation of 

 the ' reef -knolls." • 



(/) Description of the British Species of Syrinyothyris 

 and their Mutations. 



Syringothyius principalis, sp. nov. (Text-tigs. 1 a-\ c, 3 c, 

 pp. 166, 178; PI. XI, figs. 1 & 2.) 



1905. Syringothyris aft', cuspidata (Martin) — Vaughan [33] p. 215. 

 1911. Syringoth'ris typa Wmchell, Hall— Vaughan [28]"p. 364. 

 1913. Syringothyris aff. carteri — North [25] p. 401. 

 1917. Syringothyrls cf. cuspidata— Dixey &"Sibly [6] p. 131. 



In this species the shell is smaller than in any later repre- 

 sentatives of the genus : the dimensions of an average specimen 

 being — width along the hinge-line, 35 mm. ; length of brachial 

 valve, 16 mm. ; height of area, 15 mm. 



Description : — Pedicle-valve. — Area practically flat, apical 

 angle about 100°. Sinus wide, shallow, and not sharply defined by 

 the cost* on either side of it : a median line along its floor is 

 almost flat, except near the apex where it is slightly curved. 

 Lateral slopes more or less flattened. Costse fine, but distinct 

 right up to the apex, and frequently somewhat sinuous; they are 

 rounded but very depressed, and are separated by narrow groove-like 

 furrows ; there are about eighteen or twenty on each lateral slope. 

 Delthyrial supporting-plates markedly divergent ; syrinx relatively 

 large, but not deeply buried beneath the level of the area 

 (see text-fig. 1 /;, p. 166). The delthyrial supporting-plates fail to 

 extend completely across the valve in transverse sections where the 

 syrinx is most typically developed. 



Brachial valve. — There are few distinctive features in this 

 valve. The fold is sharply defined and evenly convex, but not 

 much raised above the general level of the valve : lateral slopes 

 flattened towards the cardinal extremities. The cost* resemble 

 those in the opposite valve, but are not sinuous. The beak is 

 inconspicuous, and but little raised above the hinge-line. 



Hoi o type. — A pedicle- valve from K„ subzone, Howie Hill, 

 near Ross (Herefordshire), in the collection of Principal T. F. Sibly 

 (PI. XI, fig. 2). 



Horizon and localities. — This species is especially charac- 

 teristic of the Oleistopora Zone, but extends into, and is rarely 

 represented in the lowest beds of, the succeeding horizon ; it is- 



1 Vaughan [35] p. 3. 



