58 PALAEONTOLOGY. 



tube ; some of the species have a punctate shell. The genus 

 Athyris (Dalman), not always easily distinguished from 

 Terebratula, has usually a smooth and rounded shell, orna- 

 mented with concentric lamellae or wing-like expansions 

 (fig. 12, 5); the beak is truncated by a round foramen; the 

 hinge area is obsolete ; and the spires are as in Spirifera, with 

 the addition of some further complications near the hinge. 

 There are twenty-five species, mostly from the Devonian and 

 carboniferous rocks. The species of Retzia (King) are still 

 more like plaited Terebratulce, but have lateral spires; they 

 range from the Silurian strata to the trias. Uncites gryphus, 

 (fig. 12, 6), a peculiar Devonian fossil, has a prominent beak, 

 perforated in the young shell by a minute apical foramen ; 

 the hinge-area is filled up by a deeply concave deltidium, on 

 each side of which (but only in some specimens) there is a 

 lateral pouch formed by an inflection of the margin of both 

 valves. 



The family Orthidce consists of shells with a straight hinge- 

 line, bordered by a flat, narrow area, with a central notch in 

 each valve ; the ventral valve is furnished with articulating 

 hinge-teeth, and the dorsal valve has short processes for the 

 support of the oral arms, which appear to have been horizon- 

 tally spiral (as in Atrypa). Between the oral processes there 

 is a central projection for the attachment of the cardinal 

 muscles. Internal moulds of the Orthis (fig. 13, 1) exhibit on 

 the ventral side the single attachment of the adductor muscles 

 in the centre, and on each side of it the cardinal muscles ; 

 these are surrounded by the punctate ovarian spaces and 

 impressions of the large pallial sinuses. The genus Orthis 

 includes 1 00 species, ranging upwards to the Permian, but it 

 is most abundant in the Silurian rocks. Some of the lower 

 Silurian species have a round foramen in the " pseudo-delti- 

 dium," and are called Orthisince (d'Orb.) Other species in the 

 upper palaeozoic rocks have the beak twisted or deformed, 



