ORTHOTETES. 167 



Localities. — Croyde Bay, Saunton Point, Saunton Hotel, Ashford Strand, 

 Upcott Arch Quarry, Bradiford, Poleshill, Kingdon's Shirwell, Paper Mills 

 Ilfracombe Road, Goodleigb, Top Orchard, Pouch Bridge, Rock Inn Quarry near 

 Wiveliscombe, Petherwyn. Only moderately common. 



Size. — One specimen is more than 50 mm. long and wide. 



Remarks. — Davidson considers that our Pilton fossils show no difference from 

 the Carboniferous species, 0. crenistria. It is to be observed, however, that in 

 the typical form of this species three or four smaller ribs often occur between two 

 of the larger, whereas in our specimens the larger and smaller ribs always alter- 

 nate regularly. Our ventral valves, moreover, appear to be usually flat rather than 

 concave. Hence they appear to fall within the variety araclntoidea as given by 

 Davidson, who indeed seems to refer them to that form. 



A comparison of our shells with Lummaton specimens of 0. umbraculum seems 

 to show that the distinction made between them by Davidson can most probably 

 be sustained. While both species are very variable, their ornament seems to differ 

 in character, the ribs of 0. umbraculum being relatively closer and sometimes 

 differently grouped, while the transverse threads in 0. crenistria are stronger. I 

 have not, however, seen any Lummaton specimens with the surface sufficiently 

 well preserved to show the minute ornament which Davidson describes as 

 characterising Eifel specimens of that species. 



Davidson 1 regards Orthotetes pecten, Linne, sp., as closely resembliug in size 

 and striation some shapes of 0. umbraculum. These two certainly seem more like 

 each other that they are to 0. crenistria. 



(Ehlert, 3 in a lucid and elaborate dissertation on 0. hipponyx, Schnur, 

 sp., 3 refers that shell to the Lower Devonian, 0. umbraculum to the Middle 

 Devonian, 0. crenistria, var. devonica, Keyserling, sp., 4 to the Upper Devonian, 

 and 0. crenistria to the Carboniferous, as cognate forms. From his des- 

 cription, however, Keyserling's shell does not seem to agree with the Pilton 

 form. Davidson states that it has been sometimes referred to 0. crenistria, var. 

 senilis. 5 



The Pilton shell is in itself very variable in the number and strength of its 

 ribs. A very large variety occasionally occurs (PI. XX, fig. 10) in which they are 

 very much finer and more numerous than usual (being sometimes more than 

 200), and in which the ventral valve is definitely concave. This multiplica- 

 tion of ribs may be partly but not entirely due to age. It is interesting to 



1 1865, Davidson, 'Brit. Foss. Brach.,' vol. iii, p. 78; and 1871, ibid., pt. 7, p. 306. 



2 1897, (Ehlert, ' Bull. Soc. Ge'ol. Fr.,' ser. 3, vol. xxiv, p. 856, pi. xxvii, figs. 12—16. 



3 1851, Schnur, ' Progr. d. h. Bugersch.,' p. 4. 



4 1S46, Keyserling, "Keise Petschora-Land, Geol. Beobacht.," p. 221, pi. vii, figs. 7 — 7 c. 



5 1S65, Davidson, ' Brit. Foss. Brach.,' vol. iii, p. 80. 



