70 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



has been referred to this class from as far back as the Upper Cambrian, 

 viz. Spirodentalium osceola, Walcott, but Pilsbry and Sharp consider 

 this spurious and " radically unlike any form known to belong to the 

 Scaphopoda " (^6, p. 247). The Silurian records equally fall under 

 suspicion, the tubes of the serpulid Bitrupa having frequently been 

 mistaken for Bentalium. It is not, therefore, till the Devonian is 

 reached that an undoubted member of the group is met with. 



Turning next to the big class Gastropoda, we find them foreshadowed 

 near the beginning of the Cambrian period by Scenella (a patelliform 

 shell referred by common consent to the Docoglossa), by Rhaphistoma 

 (one of the Pleurotomariidse), and by two capuloid forms, Stenotheca 

 and Platyceras, generally placed in the Capulidse. These are 

 reinforced in the Upper Cambrian by further representatives of 

 the K-hipidoglossa, viz., Murchisonia (Pleurotomariidae), Cyrtolites 

 and Oivenella (Bellerophontidse), Straparollina (Euomphalidae), and 

 Troclionema (the type of its family), and by the curious SuhuUtes, 

 which is generally referred to the Tseniogiossa. 



A certain amount of complexity attends the relationships of these 

 early Gastropods, as might be expected. Some of them are generalized 

 types : thus the Trochonematidse are considered by Ulrich {52, p. 1043) 

 to be connected through Troclionema with the Pleurotomariidae and 

 through Cydonema with the Turbinidae. 



The most difficult ones to deal with, however, are those that have 

 been considered to belong to the Taenioglossa. To begin with, such 

 very diverse forms have been placed together under one generic name, 

 especially by the earlier palseontologists. As Ulrich remarks {52, 

 p. 1068) of Platyceras, it "includes a host of wonderfully diverse 

 shells" ; and of Holopea, which appears in the Ordovician, he says it 

 " embraces much that does not belong . . . [and] most diverse 

 affinities are indicated by different sets of species, some evidently 

 being true Littorinidse, others related to Cydonema, another set to 

 Platystoma'' {52, p. 1064). 



Platyceras, as originallj^ defined by its founder, Conrad (7, p. 205), 

 contained both capuloid shells and those of a naticoid type ; and 

 though about two years later Conrad established Platystoma to receive 

 these last, the name was for a long time ignored, and any shell or cast 

 of a capuloid or naticoid character, with small spire, coming from 

 these old beds, was forthwith put down as Platyceras. The name 

 consequently carries no weight with it. Moreover, it has been rather 

 overlooked in the past that, as pointed out in my last address 

 {56, p. 248), conical shells may occur in widely different groups as 

 a response to environmental conditions : they are none of them 

 primitive forms of shell, but all the result of specialization to a 

 common end — the resistance of destructive forces. 



Reflecting on this and the fact that all these forms, including the 

 Limpets, began life with a coiled, nautiloid shell, it occurred to me 

 that the loosely-coiled forms of Platyceras might really represent 

 survivors of the ancestors of the Docoglossa, and of such Rhipicloglossa 

 as the Stomatiidae and Delphinulidae. 



Possibly this idea may have been more or less a case of unconscious 



