Shell of Zygospira Recurvirostra. 81 



opod of this form to any famity of the Rostracea or to any 

 rostrate family of the Trulacea. The presence or absence of 

 deltidial plates at maturity, however, at once indicates the sub- 

 ordinal position of any rostrate species. If the rudimentary 

 concave plate in the apex of the delthyrial cavity of nepiastic 

 Zyijospira has any phylogenetic significance, it shows that those 

 families having deltidial plates and no spondylium, the Ros- 

 tracea, had their origin in the Trullacea, a suborder in which 

 the concave plate or spondylium is functional as a muscular 

 fulcrum. This has already been inferred to be the case on other 

 grounds, as geological occurrence and complexity of structure. 

 A plate similar to that in nepiastic Zygospira exists in Gwynia, 

 Ci-stella, Atretia, and TerebratuUna, 



At the beginning of the fourth shell stage of Z. recurvirostra 

 the species is recognizable as belonging to the suborder Ros- 

 tracea and apparently most closely related to the Rhynchonell- 

 ida?. The calcareous brachial supports first appear in a specimen 

 about 1.33 mm. in length. The species is then referable to the 

 Ancylobrachia, having a loop very much like that of Centronella. 

 This loop then passes through a series of metamorphoses, acquir- 

 ing spirals when the shell is about 3 mm. in length. 



The mature exterior characters of the more prominent species 

 of Zygospira will next be considered. Z. nicoletti (figs. 11, 11, 

 12, 12a), one of the oldest species, retains many of the characters 

 of the earliest period of the fourth growth-stage of Z. recurvirostra, 

 and therefore more nearly resembles in form the primitive stock 

 which gave rise to Zygospira. In Z. saffordi (figs. 13, 13) the 

 plications are more numerous than in Z. nicoletti, but reach the 

 posterior third of the valves, and the shell is also more convex. 

 Z. reoirvi rostra is larger, more convex, with a greater number of 

 plications, which originate at the apex of the valves, and the 

 ventral beak is more strongly incurved than in the species men- 

 tioned. From Z. rcc.urrirostm one line leads through Z. uphamt 

 and Z. crrdtlra to Z. headi, comprising a group which continues 

 to increase in size, gibbosity, striation, and in the obsolescence 

 of the fold and sinus. In another phylum characterized by Z. 

 deflecta, Z. mod'rxfd, Z. cincinnatiensis, and Z. kentuckyensis, the 

 plications do increase in size but not in number, while the in- 

 conspicuous fold and sinus of Z. recurvirostra is gradually devel- 

 oped more and more strongly, so that when the extremes of 

 both lines are compared (Z. headi and Z, kentuckyensis} very 



