232 STUDIES IN EVOLUTION 



lie voit point encore les perforations tubulaire." Previous 

 to this stage, " Les lobes du manteau commencent alors a se 

 recouvrir d'une cuticule dpaisse et rigide que ne leur permet 

 plus de se mouvoir que dans le sens vertical." 



From the minuteness and the tenuous nature of the pro- 

 tegulum, its fossil preservation in an unaltered condition 

 would not be expected. Neither would it be found on the 

 beaks of mature shells, whether recent or fossil. In rare 

 cases of unusually perfect conservation of the beaks the pro- 

 tegulum is retained, but frequently its form and characters 

 are exhibited, after its removal, by the impression left in the 

 surrounding calcareous test. To study the features of the 

 protegulum, and the early stages in the growth of the shell, 

 it is very desirable and often necessary to have young and 

 well-preserved specimens. The rapid encroachment of the 

 pedicle on the ventral beak commonly obliterates, at an 

 early period, all traces of the protegulum and early nei^ionic 

 stages; while, in the dorsal valve, abrasion from foreign 

 objects, or against the deltidial covering, or the pedicle 

 itself usually removes all early lines of growth or nepionic 

 characters. In general, fully matured shells, recent or 

 fossil, do not furnish material for a study of the incipient 

 growth-stages. 



Affinities. — In looking for a jDrototype preserving through- 

 out its development the main features of the protegulum, 

 and showing no separate or distinct stages of growth, the 

 early primordial form hitherto known as Kuto7-gina Billings 

 is at once suggested. This genus, as shown below, includes 

 two distinct types, for one of which the name Paterina 

 [= Iphidea Walcott] is proposed.* 



* The strict definition of Kutorgina limits it to calcareous shells such as are 

 found near Swanton, Vermont, often occurring as casts in the limestone. The 

 original description of OboleUa chvjidata by Billings {Geology of Vermont, II. 

 948, figs. 347-349, 1861) seems to include two species. One, represented hy 

 figures 347 and 349 {he. cit.), agrees with phosphatic species having a straight 

 hinge-line as long as the width of the shell. The other, shown in figure 348, has 

 a calcareous test, shorter hinge, flattened brachial valve, and convex pedicle 

 valve with arching beak. Upon the latter species the genus was founded, and it 



