CHAPTER VI. 
THE LOWER SILURIAN LAMELLIBRANCHIATA 
OF MINNESOTA. 
BY E. 0. ULRICH. 
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- 
branchiata, which, on the whole, is an appropriate designation, has enjoyed more 
than merely temporary popularity. Of the other names, that proposed by Goldfuss 
Jn 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 Mollusca. 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 Goldfuss’ term. 
The Lamellibranchiata agree with the Brachiopoda in having bivalved shells, 
but differ 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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