248 MANUAL OF ZOOLOGY. 



Arms spirally rolled, flexible, and supported only at their origin by a 

 pair of short curved shelly processes. Shell-structure fibrous and im- 

 punctafe. 



Genera. Rhynchonella, Pentamerus, Porambonites, Sfc. 

 Fain. VI. Strophomenid(8. 



Animal unknown ; some probably free, others attached, during the 

 whole or a portion of their existence, by a muscular peduncle. No 

 calcified supports for the arms. Shell with a straight hinge-line, and 

 a low triangular area in each valve. Shell-structure librous and 

 punctated. 



Genera. Orthis, Orthisina, StropJwmena and Leptcena. 

 Fam. VII. Productidee. 



Animal unknown. - Shell entirely free, or attached to marine bottoms 

 by the substance of the beak ; valves either regularly articulated, or 

 kept in place by muscular action. No calcified support for the oral 

 appendages. 



Genera. Producta, Chonetes, Strophalosia, Aulosteges. 

 Fam. VIII. Calceolidce* 



Animal unknown. Shell probably free ; valves not articulated ; 

 ventral valve pyramidal with a large, flat, triangular area ; dorsal 

 valve flat, semicircular, with a straight hinge-line. No foramen, or 

 muscular, or vascular impressions. 



Genus. Cafceola. 

 Fam. IX. Craniad(8. 



Animal fixed to submarine objects by the substance of the shell of 

 the ventral valve. Arms fleshy and spirally coiled ; no hinge, or 

 articulating processes ; upper or dorsal valve patelliform (i.e. limpet- 

 shaped). 



Genus. Crania. 

 Fam. X. Discinidce. 



Animal attached by means of a muscular peduncle, passing through 

 the ventral or lower valve, by means of a slit in its hinder portion or 

 a circular foramen excavated in its substance. Arms fleshy, valves 

 unarticulated. 



Genera. Discina, Trenails, Siphonotreta, Acrotreta. 

 Fam. XI. LingulidtB. 



Animal fixed by a muscular peduncle passing out between the beaks 

 of the valves ; arms fleshy, unsupported by calcified processes. Shell 

 unarticulated, sub-equivalve, texture horny. 



Genera. Lingula, Obolus. 



CHAPTER XLV. 

 DISTRIBUTION OF MOLLUSCOIDA. 



DISTRIBUTION OF MOLLUSCOIDA IN SPACE. The Polyzoa, like all 

 the Molluscoida, are exclusively aquatic in their habits, but, 

 unlike the remaining two classes, they are not exclusively 



* Recent researches have thrown some doubt upon the position of 

 Calceola amongst the Brachiopoda, and have tended to show that it is a very 

 abnormal type of Rugose Coral, provided with a lid or operculum. 



