LOWER PALEOZOIC HYOLITHID^ FROM GIRVAN. 215 



Genus Plerotheca, Salter, 1852. 



The genus Pterotheca, which was established by Salter # to contain an Irish fossil 

 described by Portlock as Atrypa transversa, is represented not only by British 

 species, but also by others from America and Bohemia.t Its true position and affinities 

 have been a matter of uncertainty : Zittel, as recently as 1903, retained it in the 

 Hyolithidce, but it is believed by Ulrich and Scofield \ to belong to their family 

 Carinaropsidod, which is defined by them as follows ; — " Symmetrical, almost patelliform 

 shells, the aperture being greatly expanded : apex small, involute, overhanging the 

 posterior margin, consisting of no more than two volutions. Within the aperture a 

 broad concave septum. Anterior lip with a central emargination." The type of the 

 genus Carinaropsis § is C. carinata, Hall, of the Trenton Group, and the close affinities 

 of Carinaropsis with Bellerop>hon and Pterotheca are clearly pointed out by Ulrich 

 and Scofield. 



Pterotheca agrees with Carinaropsis in a striking manner, and possesses the 

 characteristic internal septum, the broadly expanded mouth, and the carinated back, but 

 differs by the shell being simply arcuate with an incurved beak, instead of being coiled 

 into two volutions, and also by possessing a pair of small vertical septa in the apical 

 extremity, dividing the cavity into three portions. This peculiar septate apical region 

 is considered by the same authors to be of generic importance only, but sufficient to 

 make it doubtful whether Pterotheca is a Pteropod, and should not rather be classed 

 with Carinaropsis as a family of the Docoglossa. Novak [[ considered that his genus 

 Pterygotheca somewhat recalled Salter's Pterotheca, but it is difficult to see any 

 structural affinity, or even to recognise much superficial resemblance. 



Salter's definition of the genus Pterotheca is short : — " Shell transversely oval, 

 bilobed, with wavy sides and a strong median keel. Ventral plate short, narrow, flat." 

 Only a few species have been described, and the type is Pt. transversa (Portl.).H 

 The genus is stated to occur in Bohemia and North America, and it ranges through 

 the Ordovician and Silurian. The best-known British species is Pt. corrugata, Salter, 

 1852, which is recorded from the "Caradoc" rocks of Dolbenmaen, near Criccieth.* # 

 Pt. undulata, Salter,"!"! from Carnarvonshire, has apparently never been figured or 

 described. Pt. avirostris, Salter (MS.), is from the Woolhope and Upper. Llandovery 

 Beds. 



* Salter, Brit. Assoc. Reports, 1852, Trans. Sect., p. 61. 



t Barrande, op. cit., p. 99, pi. xv. figs. 1-6. 



I Ulrich and Scofield, "Lower Silur. Gastr. Minnesota" (Geol. Nat. Hist. Surv. Minnesota, vol. iii. pt. ii., Final 

 Rep., " Palseont," 1897, chap, x.), pp. 857, 926. 



§ Hall, Palaont. New York, vol. i., 1847, p. 183. 



|| Novak, "Rev. Palseoz. Hyolith. Bohmens," Abhandl. ton. bbhm. Gese.ll. Wissen., vii., Bd. iv. (Matli.-nat. Kl.), 

 No. 6, 1891, p. 45, t. iii. figs. 36-47. 



1" Portlock, Geol. Rep. Londond., p. 455 (Atrypa transversa) ; Salter, Brit. Assoc. Rept., 1852, Trans. Sect., p. 61 ; 

 Murchison's Siluria, 5th ed., 1872, foss. 41, fig. 4, p. 199. 



** Salter, Mem. Geol Surv., vol. iii., 1866, p. 353, fig. 18. 



tt Salter, op. cit, 1866, p. 274. 



