Dr. Ivor Thomm — Devonian Fossils, Cornwall. 83 



interspace being about twice the thickness of a costa. Smaller ones 

 are occasionally interpolated. The middle of the anterior margin of 

 the brachial valve has six costse within a space of 10 mm., while the 

 maximum length of the shell is 26 mm. and the maximum width 

 41 mm. 



Second example. — This specimen does not show the costation so 

 well as the preceding, though the anterior portion is somewhat better 

 preserved. At the anterior part of the middle portion of the valve 

 there are about eleven costse within a space of 10 mm. The greatest 

 length of the preserved part of the shell, consisting only of the 

 brachial valve, is 23 mm., while the greatest preserved width is 

 52 mm. The fairly high triangular area lies in the plane of the shell, 

 while the cast of two strong delthyrial supporting plates, imperfectly 

 preserved, and an open delthyi'ium with a strong cardinal process are 

 present. The process is produced forward as a strong median septum 

 for a distance of about 16 mm. The adductor muscle-impressions 

 cover a pear-shaped area and are divided into two portions by the 

 median septum. Each of these portions is further subdivided by 

 two longitudinal grooves, another groove forming the lateral and 

 anterior boundary of each half of the muscular area. The whole 

 muscle-impression has a maximum length of 17 mm. and a maximum 

 width of 10 mm. 



Both specimens are compressed along the longitudinal axis, so that 

 the width-measurement is greater than in the original state. 



Affinities. — No described form appears to agree with this species. 

 The deformation of the examples makes comparison difficult, but they 

 suggest nearest relationship to the robust form described by Professor 

 Kayser ^ and Dr. Drevermann * from the Siegener Grauwacke as 

 Orthis personata, Zeiler. The British form, however, is more coarsely 

 costate and has a more rotund and less elongated muscular area in the 

 brachial valve. 



Locality and Horizon. — Mr. C. Davies Sherborn informs me that 

 the first-mentioned specimen was obtained by a road-mender on a road- 

 heap on the road to Grampound and about two miles south Qf Ladock. 

 Other ' shells ' were said to have been present with it on the same 

 heap. The second fossil is in undoubted Grampound Grit matrix, and 

 was found by a quarryman named Bennett in response to a reward 

 offered by Mr. tjpfield Green. Mr. Green was informed of the 

 discovery by the Ladock schoolmaster, Mr. Clemmow. The quarry 

 is on the west side of the river over the bridge from the inn. 



Though there exists a certain divergence of opinion regarding the 

 correlation of the Grampound Grit with beds of other localities, there 

 is now practical unanimity in allocating it to a Lower Devonian 

 horizon. Mr. TJssher ^ defines the term as a " name given to the 



^ E . Kayser, ' ' Beitrage zur Kenntniss der Fauna der Siegenschen Grauwacke ' ' ■: 

 Jahrb. d. K. p. geol. Landesanst., Bd. xi (1890), p. 98, pi. xi, figs. 3-5; 

 pi. xii, figs. 1-4; Berlin, 1892. 



^ P. Drevermann, "Die Fauna der Siegener Schichten, etc.": Palaonto- 

 graphica, Bd. 1, p. 264, pi. xxxi, figs. 1-8, Stuttgart, 1904. 



^ Mem. Geol. Surv., The Geology of the Country around Bodmin and 

 St. Austell, Sheet 1" 347, p. 25, London, 1909. 



