LECTURES ON MOLLUSCA. 275 



Family Spirifebjd^e. 



'In this extinct group, the mouth-arms were supported by very large 

 spiral coils, which occupy almost the whole of the sides of the shell. 

 These are sometimes spiny, showing that they were covered with stiff 

 cilia. In some members of this family the shell is pierced by tubes ; 

 in others not ; but in metamorphic rocks it is very difficult to speak 

 positively on this point. The species of Spirifera are found in palseozioc 

 rocks all over the world. They are generally very transverse, like 

 Argiope. Cyrtia has a pyramidal shape, with a prominent beak. 

 Spiriferina and Suessia include the secondary forms, with a prominent 

 plate inside the upper valve. Athyris (Spirigera) is shaped like a 

 smooth Terebratula. Merista resembles it, with arched plates round 

 the hinge. Retzia is punctured, like a Terebratulina with spiral arms. 

 Uncites is not punctured, has no hinge area, and is furnished with a 

 large concave deltidium, approaching Pentamerus. 



Family Rhynconellid^. 



Rhynconella has long, spiral mouth-arms, directed inwards, (not 

 outwards, as in the Sp>irifers^ and not supported by any shelly skele- 

 ton. The shell is not punctured, leaving the mantle loose. The living 

 species are black and slightly plaited ; the fossils are very numer- 

 ous, and generally deeply plaited, with the margin of the valves 

 twisted. In Porambonites, the surface is minutely pitted. Camaro- 

 phoria has ridges supporting dental plates. In this respect it resembles 

 Pentamerus, in which the plates are so magnified as nearly to divide 

 each of the valves. They branch in the middle, so as to inclose a 

 separate chamber in which the viscera were probably situated. Atrypa 

 resembles Rhynconella, but with the mouth-arms calcified. 



Family Orthids . 



The Orthids have punctate shells, generally very much depressed; 

 with small beaks and straight hinge. They probably had horizontally- 

 coiled spiral arms. In Orthis, the hinge-line is narrower than the shell, 

 and both valves are convex. In Orthisina, it is wider. Streptorhyncus 

 has the beak twisted. Strophomena is widest at the hinge-line. The 

 valves are nearly flat during adolescence ; when they approach matu- 

 rity, they suddenly bend to one side. Slropheodonta has a toothed 

 hinged-line. The restricted genus Leptcena has the valves regularly 

 curved. Koninckia has the valves rounded and smooth. Davidsonia 

 was attached by the outer surface of the ventral valve. Calceola is 

 generally reckoned with the " Rudistes ;" all of which are, by Philippi 

 and others, ranked with this family. It is funnel-shar-ed, resembling 

 Radiolites; but the internal markings indicate strong affinities with 

 the Orthids. The true Calceolas are a Devonian group ; the so-called 

 Carboniferous group, Hypodema, are believed to be Oapulid Gaste- 

 ropocls. 



Family Productid;e. 



In this singular group, the creatures were bent backwards ; the 

 back valve being concave, and the front valve very convex. They 



