48 Mr. R. Howse on the Permian System of the 



duced to abandon this idea and to include both forms in one 

 specific group. 



The Orthothrix Goldfussi of Geinitz is the typical form. It 

 is the Strophalosia excavata. King ; and Stroph. Goldfussi, King, 

 is without a doubt the O. excavatus, Geinitz, as any one may 

 ascertain by consulting the figures and descriptions given by 

 both these authors in the ' Versteinerungen' and ^ Monograph.' 



Stroph. Goldfussi, Miinst., is thus characterized by Geinitz : — 

 "Back (ventral) shell roundish oval, quite like the preceding 

 species ( O. lamellosus, Gein,) , and also without a sinus, and covered 

 all over the surface with close-set tubular spines and with a short 

 but high area." The shells figured by Mr. King, Mon. tab. 12. 

 figs. 13, 14, 15, 16, so evidently belong to the above description, 

 that I need not quote Geinitz further to prove it. This is the 

 form which I formerly described as Productus asperrimus. 



Geinitz's O. excavatus is described as hemispherical transverse- 

 oval, with a small, depressed, pointed umbo, a high area, and a 

 narrow, sometimes only weakly-marked sinus, with fine con- 

 centric striae, and studded with thin but long tubular spines. 

 The general character, " halbkugelig quer-oval," is so express- 

 ive of the general appearance of King's S. Goldfussi, Mon. 

 pi. 12. figs. 1-11, that I cannot doubt its identity with 0. ex- 

 cavatus. 



The regular form of this shell, 'Mon. Brit. Perm. Brach.' 

 pi. 3. figs. 19, 20, 21, 22, therefore must be considered as the 

 typical S, Goldfussi; and if it is thought desirable to distinguish 

 the other by a varietal name, excavata can be applied to it. But 

 these two forms run so much into one another, that it is im- 

 possible to draw a line of separation between them. 



The principal character by which Geinitz would separate his 

 O. excavatus from O. Goldfussi is the sinus of the ventral valve. 

 This is not a constant character, and certainly not a specific one. 

 Mr. King observes (Mon. p. 99), — "The arrangement of the 

 spines constitutes a capital distinctive character for this species ;" 

 and, strangely enough, he refers to a shell doubtfully placed in 

 Goldfussi by Geinitz and excluded from it by King as a proof. 

 Now this excluded shell with the " capital distinctive characters'' 

 is nothing more than Mr. King's S. Goldfussi, for it is the 

 O. excavatus, Gein. After all the difficulties attending the study 

 of this species, one turns with pleasure, and for confirmation, to 

 Mr. Davidson's excellent plate iii., and to the original diagnosis of 

 this shell by N. Winch in the ' Geol. Trans.' vol. iv. p. 10, " A 

 species of Donax with hair-like spines." 



This species is common in the shell- limestone of Durham, and 

 occurs also, but more sparingly, in the compact limestone, and 

 has also a very extensive lateral range. 



