BRACHIOPODA. 66$ 



example, and one of very wide geographical range, being the familiar 

 Plcctambonites (Strophomena) rhomboidalis (fig. 518), which begins in 

 the Bala beds (Ordovician), and survived into the Carboniferous period. 

 The genus Streptorhynchus [Orthotetes) comprises forms like Stropho- 

 mena, with a biconvex or concavo-convex, radially striated shell. The 

 beak of the ventral valve is long, often twisted, with a high area and a 

 pseudo-deltidium ; the area of the dorsal valve being linear. The genus 

 ranges from the Devonian to the Permian, characteristic species being 

 the S. umbraculum of the Middle Devonian and the S. crcnistria of the 

 Carboniferous. 



In the genus Leptcena are forms smaller than the majority of the 

 Strophomena", but in many respects resembling these. The shell is 

 more or less completely semicircular (figs. 514, e, and 519), trans- 

 versely elongated, with a double hinge-area, notched in the centre, 

 the fissure in the ventral valve having a deltidium, 

 and the surface being generally striated. Speak- ^r-"^ : r^ 

 ing generally, the species of Leptcena can usually 

 be separated from those of Strophomena or 

 Orthis by the form of the transversely elongated 

 shell, the valves of which are strongly bent, so 

 that one (the dorsal) is always very concave, and 

 the other (the ventral) correspondingly convex. iceaf'a^Y^n.tMv^-,^, 

 Moreover, the muscular impressions are elon- ^PJJ 1 d J2j ve brdcfvkiin" 

 gated instead of being laterally expanded, as 

 they are in the genus Strophomena. The genus Leptcena ranges 

 from the Ordovician to the Carboniferous. A few forms from the 

 Lower Jurassic rocks have also been referred to the genus Leptcena, 

 but some of these are now placed in the succeeding family, in the 

 genus Koninckella, and it is very doubtful if the others are properly 

 referable to Leptce.na. 



Finally, the genus Davidsonia includes certain singular Brachio- 

 pods, in which the shell is thick and solid, and is attached to foreign 

 bodies by the substance of the ventral valve. The ventral valve has 

 a wide area, with a triangular fissure covered by a convex deltidium ; 

 and though there are no calcified brachial supports, the position of 

 the arms is indicated by two spirally-grooved elevations in the in- 

 terior of the valve. The species of Davidsonia are found in the 

 Devonian rocks, and the genus should perhaps be referred to the 

 following family. 



Family 3. Koninckinid.^e. — This family is incompletely known, 

 and includes small Brachiopods, in which the shell is plano-convex 

 or concavo-convex, the hinge-line being straight or curved, and a 

 hinge-area being absent. There may or may not be a foramen in 

 the beak of the ventral valve, and the arms are supported upon two 

 loosely-inrolled spiral lamellae, the apices of the coils being directed 

 towards the ventral valve. The typical forms of this family range 



