224 CAMBRO-SILURIAN FOSSILS. 



Strophomena spiriferoides (M'Coy). 



Sp, Char, Subrhomboidal, sides rather depressed, only slightly 

 convex ; hinge-line as wide as the shell ; cardinal angles square 

 or slightly acute, sides subparallel, slightly rounded, front very 

 abruptly raised into a quadrate sinus, one-third wider than 

 high, from which a very prominent rotundato-quadrate inesial 

 ridge extends sharply defined to the beak, and an almost 

 equally deep and strongly defined flattened mesial hollow ex- 

 tends to the beak of the receiving valve, both valves radiated 

 with subequal thread-like ridges, occasionally dichotomising 

 (about fourteen in three lines at six lines from the beak) : casts 

 of the receiving valve show the closed, cicatrized foramen, with 

 a long slender cylindrical extension of matrix, half a line thick, 

 arching from the apex of the beak into the back, at one-fourth 

 the length from the beak, and two thick, short, slightly di- 

 verging cardinal teeth and lamellae ; casts of entering valve 

 show two very short, thick, diverging pits of cardinal teeth, 

 and a large double ovate pit left by the great bifid rostral 

 tooth, and a trace of a slender mesial septum. Average width 

 1 inch, proportional length of receiving valve -^q^q, of entering 

 valve j%%, depth ^%%. 



This is so extremely like the Spirifera radiata of the Wenlock 

 limestone, that I have no doubt it has very often been confounded 

 with it, and probably all the examples quoted by authors of S. 

 radiata from the Caradoc and inferior strata will be found re- 

 ferable rather to the present shell, which abounds in these infe- 

 rior rocks, where I have never seen the true S. radiata (nor does 

 the present species occur to my knowledge in the Upper Silurian 

 strata) . Externally the present shell may be distinguished from 

 the S, radiata by its longer hinge-line, flatter sides, and coarser 

 striation, while the internal casts show the generic diff*erence by 

 demonstrating the pit for the great rostral tooth in the beak of the 

 entering valve, and the slender tubular filling of the apical fora- 

 men as in the Leptcena [Strophomena) sulcata (Vern.), Bull, de 

 la Soc. Geol. de France, 2nd S. vol. v. p. 31 . f . 4, to which it is 

 most nearly allied, but from which it diff'ers by its greater gib- 

 bosity, and very prominent mesial ridge and hollow extending 

 from the beak, giving it exactly the appearance of Spirifera. The 

 Orthis Vespertilio when very finely sulcated might be mistaken 

 for this species, but has the mesial depression in the entering and 

 mesial elevation in the receiving valve, in which also the trian- 

 gular foramen is open throughout. 



Extremely common in the impure limestone of Moel y Garth, 

 Welchpool, Montgomeryshire ; very abundant in the limestone 



