184 SUPPLEMENT TO THE 



Hicks, who, in his able researches in the neighbourhood of St. David's, had found that 

 0. Carausii was one of the characteristic fossils of the Tremadoc Group. 



Mr. Salter was not, however, very far wrong when he considered Orthis Carausii to be 

 a dwarf variety of 0. calllf/ramma, for it is nearly related to Dalman's species, and 

 certainly is referable to the 0. calU(jramma group and not to the 0. AcionicB group, although 

 differing from Dalman's species sufficiently to make it desirable to retain for the Tremadoc 

 fossil a distinctive specific designation. It is also a smaller shell, and its dorsal valve 

 slightly less convex than in true 0. callirjramma. It shows a slight mesial longitudinal 

 depression in the dorsal valve, but this feature is also observable to a greater or lesser 

 extent in 0. calligramma. 



103. Orthis Hicksii, Salter. Dav., Sil. Mon., PI. XXXIII, figs. 13— IG. 



Orthis Hicksii, Salter. A Catalogue of the Collection of the Cambrian and Silurian 

 Fossils in the Museum of Cambridge, p. 8, 1873. 



As already stated, Prof. Hughes has expressed the opinion that Orthis Carausii and 

 0. Hicksii were referable to a single species. After a careful examination of some 

 specimens of 0. Hichsii sent to me by Dr. Hicks in 1868, I arrived at the conclusion 

 that they were specifically separable from 0. Carausii, an opinion I still maintain. 0. 

 Hicksii is a smaller shell when adult, and has a comparatively fewer number of ribs ; the 

 surface of each valve is ornamented with about ten principal ribs, with very often a short 

 intermediate rib, and this is a good distinctive character between it and 0. Carausii. While 

 alluding to this species in the ' Cambridge Museum Catalogue,' Mr. Salter says : " Very 

 like 0. Carausii, Salter, but with a short hinge." Mr, Salter has evidently overlooked its 

 real distinctions, namely, that of possessing a short intermediate rib between the longer 

 ones. 



Dr. Hicks, who has seen almost all the specimens of this shell that have been collected, 

 and who has again recently visited the Menevian Beds, in the Porthyrhaw Valley, St. 

 David's, to obtain more specimens, writes me that, in his opinion, it and 0. Carausii are 

 distinct species ; and that, as they come from very distinct horizons and are associated with 

 faunas separated by several thousands of feet of strata, without a single species in common, 

 it seems hardly likely that they are the same. OrtJiis Hicksii at St. David's occurs in 

 the upper part of the Menevian group, 0. Carausii in the Tremadoc group ; these two 

 groups being separated by the whole of the Lingula-flags (including the Maentwrog, 

 Dolgelly, and Efestiniog groups, each with distinct faunas).^ It is consequently a much 

 older species than is 0. Carausii. Unfortunately 0. Hicksii is a scarce fossil, and it is 



1 See Dr. Hicks' paper, " The Classification of the Eozoic and Lower Palaeozoic Rocks of the British 

 Isles," ' Popular Science Review,' new series, vol. v, 1882. 



