118 



BRITISH SILURIAN BRACHIOPODA. 



septum, and are separated one from the other by smaller obUque ridges. Two specimens 

 measured — 



Length 12, width 11 lines. 

 „ 8, „ 9 „ 



Obs. This shell does not appear to have been well understood by those who have 

 described it, nor can the genus to which it belongs be yet with certainty determined. It 

 is not a Spirifer, as Prof. M'Coy and one or two others have supposed it to be ; for, 

 besides the interior arrangements not being those of that genus, it possesses no area ; the 

 lateral portions of the beak being regularly rounded off from the margin of the fissure. 

 The fissure itself is also in a great measiu'e filled up by the incurved beak or umbone of 

 the dorsal valve. Then there appears no sufficient or valid grounds that I can perceive 

 for placing it with Porambonites, nor with Atrypa ; and consequently the genus to which 

 it bears most affinity would appear to be Meristella or Athyris. At p. 230 of ' Siluria' 

 (2nd edit.) Mr. Salter states that Atrypa crasm is probably of the same genus as the 

 so-called BJiynchonella angustifrons, and that Terebratula furcata, Sow. (Min. Conch., 

 PI. 21, fig. 16), one of the same group, is an Upper Llandovery species. Now, I have 

 already shown that the so-called Hemitliyris or Rhynchonella angustifrons of M'Coy and 

 Salter is either a Meristella or an Athyris, and is possessed of spiral lamellae for the 

 support of the oral arms, as in those genera ; and consequently it is highly probable that 

 Meristella crassa was similarly provided. 



My description of the interior is taken from gutta-percha moulds, made from internal 

 casts ; but, as casts alone have been hitherto found, I will here reproduce the description 

 Prof. M'Coy has given of them — " Casts of the receiving (ventral) valve show two very 

 strong slits of the dental lamellae, converging so as to meet in the rostral part, enclosing 

 the large triangular foramen, and again diverging in their anterior part, reaching to 

 within one fourth the length of the anterior margin : each side with about six thick 

 radiating ridges (ovarian?), and tAvo or four on the middle lobe, all obscurely forking at 

 the margin. Casts of entering (dorsal) valve show two thick diverging dental pits at the 

 beak, a wide shallow mesial depression, with an obscure cmcial arrangement of four 

 angular ridges defining the muscular impressions." 



I have not myself seen a perfect bivalve specimen of this common species, and the 

 figures given in the ' Silurian System' are very imperfect. Although I have not been able, 

 from want of sufficient material, to offer complete figures of the exterior, I believe those I 

 have given of the interior may be considered an improvement on those hitherto published, 

 and will assist the student in any further researches he may make with reference to the 

 species. 



Position and Locality. Meristella crassa is common in the Lower Llandovery, and 

 rare in the Upper Llandovery or May Hill rocks. Mr. J. de C. Sowerby describes it 

 from Cefn Rhyddan, Llandovery. Mr. Salter mentions it from the Upper Llandovery 

 rock of Bograine, near Shelve ; and from the Lower Llandovery of Cyrn-y-brain, and of 



