980 THE PALEONTOLOGY OP MINNESOTA. 



LKiipbistninlclif. 



Subclass PROSOBRANCHIATA. 



Order PECTINIBRANCHIATA. 



Suborder EOTOMACEA. 



This proposed suborder is made up chiefly of four families of shells that are 

 perhaps the most important of all to the paleontologist, namely, the Rapliistomidce, 

 Pleurotomariidce, Euomphalidce and Trochidce. Besides these it should include other 

 families, as for instance the Fissurellidce and Haliotidce, which were most probably 

 derived from the Pleurotomariidce, provisionally also the Maclureidce because of their 

 evident relations to the Euomphalidce. Then we include also the Turbinidce because 

 their early Paleozoic prototypes can be shown to have very close relations with 

 unquestionable members of the group, while of their recent representatives it is 

 well known that they are not greatly different from the Trochidce. 



We find it most difficult to designate the characters of the shell which may 

 fairly be said to be peculiarly characteristic of the group. Perhaps such characters 

 do not exist, at any rate we shall not now attempt to point them out. For the 

 present it must suffice to say that the suborder rests principally upon observations 

 which we regard as proving the common origin of the four families first mentioned. 

 These observations will appear in the course of our remarks on the families known 

 to have representatives in the Lower Silurian rocks of Minnesota. 



Family RAPHISTOMIDJ], n. fam. 



This family includes shells which we regard as the best known representatives 

 of the original stock from which the Euompholidce, Pleuroiomariidw and Trochida: 

 were almost simultaneously evolved. The position of the majority of the forms is 

 intermediate between the first two families, leaning toward the second rather than 

 the first, while the rest compare better with types that we place as early representa- 

 tives of the last family, particularly with certain of the Upper Silurian shells which 

 Lindstrom refers to the recent genus Trochus. 



The most persistent character of the Raphistomidw is one that at first may seem 

 almost trivial, but because of its persistency it is justly entitled to rank as 

 important. Namely, the lines of growth on the upper side of the whorls, which of 

 course correspond in direction with the outline of the upper lip of the aperture, 

 though directed on the whole backward, are curved sigmoidally, thus causing a 

 usually very slight sinus in the outer part. The curvature is never strong and is 

 perhaps best developed in Raphistoma in which the point at which the change in 



