taj 



in error when he denies homology between the carina of the group of /'/. ,ilnt,i 



'i/il.itofilfnt.^ ami of Unphistoma aii-l /.'"/>/M/owia on the one hami. and the 

 " collar" of !:> til ii>tenu on the other. On later pages of his valuable work (488, ! 

 Koken compares the genus BNMyAoJppfcnM with l>'l/iliinuln and particularly with 

 the so-callo'l >V./n'ni r<iill(iwli<inutn of d'Orbi^ny. a Mesozoic shell. The agreement 

 of the latter with A., alutus 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 

 a././ ;iml (hni- ran 'lerive.l. like A'I<M;/I.///</-M, from descendants of 



In his most exemplary work on the "Silurian Gastropoda of Gotland " Lindstrom 

 describes and most beautifully illustrates a large number of shells which he refers to 

 Trockui. A comparison of the figures, excepting T. pr<tntn<hm and T. cavus, is 

 calculated to give the impression that the whole assemblage represents numerous 

 ami very diverse specific modifications of a single generic type. And yet it is 

 possible to pick out several groups t li.it may be brought into very plausible connec- 

 tion with widely different Lower Silurian types. Thus, the group of which T. 

 the central form, with T. nxtniliformis and perhaps T. stuxbergi and T. 

 ijnihliimlicMs on one side and 7'. inrisut on the other, we regard with much confidence 

 as derived from Raphistominn ; T. wisbyensis, T. Inmellnstix, T. fulminatiis and T.dnlli 

 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 

 impre-.-,.,! 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 7'w//nx-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 Trochidn- and 

 Ttirlnniiln have been derived from an early type oiRaphiKtuminn, considers them as a 

 great .h-i'I.iy 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 a >tock. According to the second 



explanation the groups of species mentioned in the pn-ciiing paragraph are actual 

 descendants respective! '../i. <./. \\ hich, because 



they lived under the sanx- ciin<lition<.. or for some other unknown cause, assumi"! 

 -imilar characters with their neighl>ors, the gradual convergence of characteristics re- 

 sulting inaseriesof f"im- that to many may seem almost inseparable. liothof these 

 explanations are theoretically correct, ami although we are inclined to accept the 

 second as the most rational, it is not at all improbable that the truth lies l..-t ween 

 them. 



