LECTURES ON MOLLUSCA. 127 



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 palceozioc 

 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. Atliyris (Spirigera) is shaped like a 

 smooth Terebratula. Merista resembles it, with arched plates round 

 the hinge. Retzia is punctured, like a Terebralulina 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 Spirifers,') 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 

 P entamerus j 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 ORTIIIDJE. 



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. Slreptorliyncus 

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

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

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

 hinged-line. The restricted genus Leptwna 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-shaped, 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 Capulid Gaste- 

 ropods. 



Family PRODUCTID^:. 



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

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

 were probably attached by the long hollow spines, which adorn the 

 shells ; and may have moored themselves in chinks, or partly buried 

 in mud. Productus has the hinge-line linear, and is a Devonian 

 group. Aulosteges has a hinge-area, like Spondylus. Strophalosia 

 was attached by the beak of the front valve. The Silifrian Chonetes 

 has one row of spines along the hinge-line of the front valve. 



