104 THE NAUTILUS. 



the differing intensity of the sun's rays as a partial explanation of 

 these surprising differences. 



This same author has protested against any sweeping conclusions 

 as to the importance in the operculate shells of the operculum, in es- 

 tablishing genera, and agrees with Petit de la Saussaye in consider- 

 ing this feature of quite subordinate value, remarking, " porque varia 

 el operculo de los Cyclostomas en los grupos mas aproximados por 

 el caracal y las costumbres del animal. No suce de asi en los tur- 

 bineos, entre los cuales se nota que los que viven apegados a los 

 arrecifes tienen el operculo corneo, y los que se apartan a mayor 

 profundidad lo tienen calcareo : de aqui un excelente caracter para 

 dif'erenciar las Litorinas de los verdaderos turbos y trocas." 



Poey has also pointed out the confusion that results from the 

 adoption by different authors of varying or diverse morphological 

 characters for the classification of this group. 



A Cyclostoma becomes a Chondropoma because of a corneous 

 operculum, or the same shell a Megalostoma by its thickened peris- 

 tome, a character participated in by the Cyclop/torus of Montfort, 

 and the Tropidophora of Troschel. 



It is perhaps interesting to recall that Poey in a protest against 

 classification based upon the shelly parts of these mollusca, has 

 called attention to the method of progression of the Cyclostomas, 

 their divided pedal muscle allowing the movement forward of one- 

 half of the foot alternately with the other. It is not at all likely 

 that this feature would ever be useful in separating the round- 

 mouthed shells, but Poey has made of this observation an opportunity 

 to emphasize the preponderant significance of the physiology of mol- 

 luscs over the form, etc., of their shells for purposes of classification. 

 Cyclophorus has not a divided foot. Pfieffer, in his Monographia 

 Pneumonopomorum Viventium, analyzes the Cyclostomacea as com' 

 posed of three series, in each of which the form of the operculum is 

 regarded as the separative character. The entire systematic table 

 is as follows, so far as the opercula are here discussed. 



FIRST SERIES. 



Operculum circular, frequently concave exteriorly, formed of many slowly- 

 increasing whorls. Nucleus central. 



I. Operculum shelly (testaceous). 



Genera, Gyclotus, Alycseus, Diplommatina. 



Oyclotus. Operculum orbicular, shelly, arctispiral, externally rather con- 



