BRITISH SILURIAN BRACHIOPODA. 183 



Soon after the publication of the above-named paper, Prof. Hughes kindly sent me a 

 large series of the specimens of OrtJds Carausii which he had collected at Bone, Tyhen, 

 E. of Treiowerth, Prysowenfach, and Caervan in Anglesey, adding that these specimens 

 ■were procured from the basement bed of the Cambrian of Anglesey, and that it is only 

 a few feet above Archaean (Pre- Cambrian). All the specimens occur in the condition 

 of impressions of the exterior of the shell and of internal casts of both valves, and are in 

 a very good state of preservation. It was therefore quite easy by the means of softened 

 white gutta-percha to reproduce the exterior and interior of both valves, and which for 

 descriptive purposes are quite as good as if the shell itself had been preserved. These 

 specimens agree well with the figures published by Salter in pi. xxii of the ' Geology 

 of North Wales.' They occur in immense numbers in a light or dark yellow, brownish 

 or dark grey sandstone or grit in the Anglesey localities, and both rocks and casts are 

 exactly similar to those that had been previously collected by Dr. Hicks from similar 

 light yellow and dark earthy flags and flaggy sandstones in the Lower Tremadoc group 

 at Tremanhire, St. David's, in Pembrokeshire, and from which the figures 1 to 5 and 7 of 

 PI. XXXIII of my ' Silurian Monograph ' were taken. 



After a most careful study of the casts and impressions of 0. Carausii sent to me 

 hy Prof. Hughes and Dr. Hicks, I could discover but one type of Orthis, all the 

 specimens being referable to a single species, none of the specimens belonging to Orthis 

 Hicksii. None of the examples of 0. Carausii exceed some 7 lines in length by 9 

 •in breadth, most of them being of smaller dimensions. Some individuals were more 

 transversely semicircular than others, and the hinge-line is as long as or shorter than the 

 •breadth of the shell. In all the specimens of the exterior of both valves the ribs were 

 ^im^le and without any intervening shorter rib. These ribs varied in number in different 

 individuals ; thus in a specimen measuring 9 lines in breadth, fifteen strong ribs could be 

 counted with concave interspaces of about equal breadth ; in another specimen, 5 lines in 

 length by 7 in breadth, the ribs were eighteen in number ; the whole surface of both 

 valves being likewise intersected at close intervals by fine equidistant, concentric, projecting 

 lines. 



I have nothing to add to what I stated in my ' Monograph ' with respect to the 

 interior of the valves, but I have given enlarged figures of internal casts from specimens 

 sent to me by Prof. Hughes. The ventral valve of 0. Carausii is evenly convex and 

 deep, the dorsal one but slightly convex, with a shallow, longitudinal depression along 

 the middle. 



It was rightly observed by Prof. Hughes that Salter, in 1866, and at p. 258 of the 

 ' Memoirs of the Geological Survey of Great Britain, refers the Orthis under description 

 from Treiorwerth, Anglesey, to a dwarf variety of 0. calligramma, and it was only 

 subsequently that he gave to the shell the distinctive MS. name of Carausii. This he 

 did when he discovered that the rock containing the fossil was of a much greater age 

 than that of the Caradoc, where he had placed it, and which discovery was due to Dr. 



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