GASTROPODA. 939 
Raphistomida.] 
in error when he denies homology between the carina of the group of Pl. alata (i. e. 
Euomplalopterus) and of Raphistoma and Raphistomina on the one hand, and the 
“collar” of Lecyliopterus on the other. On later pages of his valuable work (438, 439) 
Koken compares the genus Hwomphalopterus with Delphinula and particularly with 
the so-called Solarium catllaudianum of d’Orbigny, a Mesozoic shell. The agreement 
of the latter with EH. alatus is exceedingly close, and we are quite willing to admit 
the justice of his comparisons because they accord so well with our view that the 
Trochide and Onustide were derived, like Huomphalopterus, from descendants of 
Raphistomina. 
In his most exemplary work on the “Silurian Gastropoda of Gotland” Lindstrém 
describes and most beautifully illustrates a large number of shells which he refers to 
Trochus. A comparison of the figures, excepting 7. profundus and T. cawus, is 
calculated to give the impression that the whole assemblage represents numerous 
and very diverse specific modifications of a single generic type. And yet it is 
possible to pick out several groups that may be brought into very plausible connec- 
tion with widely different Lower Silurian types. Thus, the group of which T. 
lundgrent is the central form, with T. astraliformis and perhaps T. stuxbergi and T. 
gothlandicus on one side and T. incisus on the other, we regard with much confidence 
as derived from Raphistomina; T’. wisbyensis, T. lamellosus, T. fulminatus and T. dalli 
remind one in all respects, excepting that their apertures are more oblique, of some of 
the smaller species of Trochonema figured in this work; finally we are so greatly 
impressed with the similarity between T. mollis and our Cyclonema transversum, 
that we can scarcely concede that they are not genetically related. 
Now, with respect to these Gothlandic Trochus-like shells, the closeness of the 
resemblance existing between them may be explained in two different ways. The 
first, starting from the almost demonstrable assumption that the Trochide and 
Turbinide have been derived from an early type of Raphistomina, considers them as a 
great display of varietal or specific modifications of a single type, the varieties 
severally taking on more or less of the distinctive features of previously established 
lines that had their origin in the same ancestral stock. According to the second 
explanation the groups of species mentioned in the preceding paragraph are actual 
descendants respectively of Raphistomina, Trochonema andCyclonema, which, because 
they lived under the same conditions, or for some other unknown cause, assumed 
similar characters with their neighbors, the gradual convergence of characteristics re- 
sulting in aseries of forms that to many may seem almost inseparable. Both of these 
explanations are theoretically correct, and although we are inclined to accept the 
second as the most rational, it is not at all improbable that the truth lies between 
them. 
