314 Davidson — Earliest British Brachiopoda, 



the earliest species of the genus Orthis, since they will be very shortly 

 minutely described and illustrated in the third portion of my Silurian 

 Monograph now in the press. Orthis is one of the earliest genera of 

 Brachiopoda at present known, for it is represented by a single 

 species in the rocks of the Menevian period. It is followed up by a 

 fresh species in the Upper Lingula flags, and by other two new 

 forms in the Arenig, or Lowest Llandovery period. 



Orthis Hicksii, Salter, M.S. PL XVI. Figs. 17-19. 



Shell small, measuring about four lines in length by five in breadth, 

 transversely oval, hinge-line shorter than the greatest breadth of the 

 shell, cardinal angles rounded. Dorsal valve semicircular, mode- 

 rately convex, slightly longitudinally depressed along the middle. 

 Ventral valve convex, deeper than the opposite one. Area triangular, 

 moderately wide. Surface of valves ornamented by about ten prin- 

 cipal narrow radiating ribs, with wide interspaces between each pair, 

 in the middle of which is situated a shorter' rib. 0. Hicksii is a 

 scarce fossil, and very rarely found in a complete condition. It was 

 discovered by Mr. Hicks in the Middle (Sandstone) beds of the 

 Menevian Group, at Ninewells and Porth-y-rhaw, near St. David's, 

 and is the oldest species of the genus on record. It was named 

 after Mr. Hicks by Mr. Salter, but was not figured or described. It 

 has not been found as yet in North Wales. 



Orthis lenticularis, Dal. sp. PI. XVI. Figs. 20-22. 



This small species has been described at some length by Mr. Salter, 

 at p. 339 of vol. iii. of the Memoirs of the Geological Survey of Great 

 Britain. It occurs in the Upper Lingula flags (Dolgelly group of 

 Mr. Belt), at Ogof-ddu, Criccieth, near Portraadoc; Penmorfa 

 Church, Tremadoc; at Gwerny-y-Barcud, Ehiwfelyn, and in several 

 other Welsh localities. It is immensely abundant in some beds, but 

 usually of very small dimensions, and very much distorted from the 

 effects of cleavage, so that it is all but impossible to obtain a complete 

 example. 



Orthis menapi^. Hicks, M.S. PI. XVI. Figs. 24-28. 



Shell truncate-orbicular, rather wider than long; hinge line a 

 little shorter than the greatest width of shell. Dorsal valve semi- 

 circular, slightly convex, with a longitudinal depression or sulcus. 

 Ventral valve rather deeper than the opposite one, with a longi- 

 tudinal ridge commencing at the extremity of the small incurved 

 beak, and gradually widening as it nears the front. Area rather 

 narrow. Surface of both valves covered by numerous fine bifurcat- 

 ing striee. About seven lines in length by eight or nine in width. 



O.-menapice is a well-marked species, but is nearly alwaj's more or 

 less out of shape from the effects of cleavage and fossilization. It 

 was discovered for the first time hy Mr. Hicks at the base of the 

 Arenig group, or Lowest Llandeilo, at Tremanbire, and subsequently 

 at Llanveran, Ramsey Island, and Whitesand Bay, all near St. David's. 

 The designation menapice, assigned it by Mr. Hicks, is the classical 

 name for the earliest city mentioned in the St. David's district. It 

 occurs in company with the following species, but is easily distin- 

 guished from it by its shape, as well as by the number and strength 

 of its striae. 



