1907.] natural sciences of philadelphia, 209 



Morphology. 



Among the species of Pyrida (as here restricted) the similarity of 

 their grosser morphologic features is evident at a glance. The shell is 

 always light and thin, the whorls are as a rule very evenly rounded and 

 always increase rapidly in size, the spire is low and each whorl rises 

 high on its predecessor. This latter feature often makes the top of 

 the shell appear quite flat. Contrasted with the low spire, the branchial 

 siphon is much produced anteriorly and often slightly curved. The 

 shell is, in fact, roughly pear-shaped. ' . . 



It is, however, only when the finer features of the ornamentation are 

 studied that we realize the unusual constancy of the genus. The early 

 whorls are smooth, and this smooth stage may occupy from one (or 

 even less than one) to two and three-quarters or even three whorls.^ 



The smooth stage is followed rather sharply by the fine ribs and 

 spirals of the cancellated stage. Perhaps, on the whole, the spirals 

 appear a little earlier in life than do the ribs, but this is not a marked 

 feature. The cancellated stage persists in all the species to the end of 

 shell growth. At first there is but one set of spirals, but later secondary 

 sets appear between the primary spirals. Still finer sets of spirals are 

 introduced with the course of the ontogeny. Sometimes by the end of 

 the last whorl the secondary and later spirals may have acquired about 

 the same strength as the original primary spirals, though it is more 

 common for the primary spirals to be strongest, with the other sets 

 fainter and fainter according to the order in which they are introduced 

 into the ontogeny, the last sets introduced being faintest. On their 

 first appearance the ribs and the primary spirals are of about equal 

 strength, but in the later whorls the primary spirals are as a rule consid- 

 erably stronger than the ribs. Exceptionally the ribs on the later 

 whorls may be as strong as the primary spirals. 



To give any idea of the morphologic features which now serve, and 

 have in the past served, to differentiate the species we must employ 

 unusual methods. Ordinary descriptions fail to convey the meaning. 

 It has therefore been thought best to use tables for the purpose. In 

 these tables the two major ontogenetic stages are represented by the 

 letter A for the smooth stage and B for the cancellated stage. These 

 stages are marked off on the line of the ontogeny as they appear in the 

 different whorls. The nimibers stand for the whorls. 



' Sections of the apex iu P. papijratia Saj', show that it is filled with a secondary 

 limy deposit. 



