CHAPTER VI. 



THE LOWER SILURIAN LAMELLIBRANOHIATA 



OF MINNESOTA. 



BY B. O. ULKICH.. 



A number of names for this class of mollusks, commonly known as mussels, 

 have, from time to time, been proposed, but none of them, save Blainville's Lamelli- 

 hranchiata, which, on the whole, is an appropriate designation, has enjoyed more 

 than merely temporary popularity. Of the other names, that proposed by Goldfuss 

 in 1820, Pelecypoda, alone presents fair claims to recognition, since its adoption would 

 produce that most desirable element, uniformity, in the terminology of the various 

 classes comprised in the subkingdom MoUusca. Blainville's name, however, has six 

 years' priority, and is so well established in literature that it is doubtful if the con- 

 fusion which would result from a change of names would be sufficiently compensated 

 for by the superior advantages of Groldfuss' term. 



The Lamellibranchiata agree with the Brachiopoda in having bivalved shells, 

 but diflfer in having them, as a rule, equal and inequilateral instead of inequivalved 

 and equilateral; they are, furthermore, placed on the sides of the animal (for which 

 reason we distinguish them as right and left), instead of above (dorsal) and below 

 (ventral). From the Gastropoda and Cephalopoda they are distinguished by wanting 

 a distinct head, in having bivalved shells, a bilobed mantle and lamelliform gills 

 developed in pairs. 



Generally the animal is symmetrically developed, of oval, rounded or trans- 

 versely elongate form, laterally compressed and enclosed in the two fleshy, often 

 more or less united, lobes of the mantle. Within the latter, which are attached to 

 and secrete the calcareous or perlaceous valves, we have first the lamelliform gills, 

 and between these the various internal organs, such as the heart, intestines and 

 organs of generation, and the mouth and anal opening, and usually also a protrusible 

 muscular foot. Numerous modifications of the mantle lobes occur. Sometimes they 

 are separate, at other times their margins are grown together so as to enclose the 

 animal as in a sack. In the latter case an opening is left in front for the protrusion 



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