218 D. M. S. Watson — A New Genus of Brachiopods. 



dorsal valve with a continuous margin. There is, however, no 

 difficulty in supposing that the entire margin has secondarily 

 developed by a fusion of the edges of the pinnae of Oldhamina 

 analogous to that which we have supposed to have formed the " septa" 

 of the ventral valve of that form. I am therefore disposed to refer 

 Pterophloios to the Lyttoniidse, a family whose definition will then 

 become : — 



Family LrnoKriiD^:. — Protremata cemented to a foreign object 

 either throughout life or in youth by a ventral valve with area, 

 delthyrium, and teeth vestigial or absent, with a flange on which 

 the edge of the dorsal valve fits. Dorsal valve smaller than ventral 

 and with a deeply slit margin, there being a single anterior median 

 and paired lateral slits. The slits in the dorsal valve and the folds in 

 the flange of the ventral valve which correspond to them may close 

 up so as to form false septa, and produce secondarily an entire margin 

 to the dorsal valve. The muscular insertions on the ventral valve 

 are long, narrow, rather widely separated anteriorly, and may be very 

 unsymmetrically developed. 



Genus Poikilosakos, gen. nov. — Lyttoniids with the ventral valve 

 cemented throughout life by its whole area. Only two pairs of 

 lateral slits in the dorsal valve. Muscles asymmetrically developed, 

 the abductor of one side and the adductor of the other alone being- 

 functional. Type, Poikilosakos petaloides, sp. nov. Cisco Beds 

 (Uralian) : Graham, Texas. 



Keyserlingina, Tschernyschev. — Lyttoniid with the ventral valve 

 strongly convex, cemented only by part of its area. Only two pairs 

 of lateral slits in the dorsal valve. Type. It. Uralian : Timan, 

 Russia, and the Carnic Alps. 



Lyttonia, Waagen ( = Leptodus, Richthofen). — Lyttoniids with 

 very large, narrow, flat ventral valve, cemented only by the umbonal 

 region. From ten to twenty-five pairs of lateral slits in the dorsal 

 valve in adults. Folds in flange of ventral valve fused to form 

 "septa". Muscles nearly symmetrical. Artinskian and later : Salt 

 Range, India; Lo-ping, China; Guadaloup Mountains, Texas; 

 Sosiokalk, Sicily ; ? Timor. 



Oldhamina, Waagen. — Lyttoniids with large, very convex, involute 

 ventral valves, attached by the umbonal region in youth, subsequently 

 becoming free and developing a callus over the posterior end. Very 

 many pairs of lateral slits in the dorsal valve. Folds in flange of 

 ventral valve fused to form septa. Post- Artinskian: Salt Range, 

 India. 



Pterophloios, Giimbel. — Small Lyttoniids with a convex ventral 

 valve cemented only in youth by the umbo. About seven pairs of 

 lateral slits in the dorsal valve. Edges of slits in the dorsal and 

 folds in the flange of ventral valve fused so as to form septa, and 

 restore an entire margin to dorsal valve. Pterophloios emmerichi, 

 Giimbel. Rhsetic : Kossen, Austria. 



Beyond the fact that they were undoubtedly members of the 

 Protremata, it is impossible to gain any idea of the ancestors of the 

 Lyttoniidas. It is natural to suppose that their ancestors retained an 



