Shell of Zygospira Iiecnrvirostra. 81 



o]jod of this form to any family of the Rostracea or to any 

 rostrate family of the Trulacea. The presence or ahsence of 

 (leltidial plates at maturity, however, at once indicates the suh- 

 ordinal ])osition of any rostrate species. If the rudimentary 

 concave plate in the a})ex of the delthyrial cavity of nei)iastic 

 Zygospira has any phylogenetie significance, it shows that those 

 families having deltidial i)lates and no spondylium, the Ros- 

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

 the concave plate or spondylium is functional as a muscular 

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

 grounds, as geological occurrence and complexity of structure. 

 A plate sinailar to that in nepiastic Zi/ffospira exists in Givynia, 

 Cistelld, Ati'ciia, and Terfhralidina. 



At the beginning of tlie fourth shell stage of Z. recur virnsira 

 the S2)ecies is recognizal)Ie as belonging to the suborder Ros- 

 tracea and apparent]}^ most closely related to the Rhynchonell- 

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

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

 Ancylol)rachia, having a looj) very much like that of Centronella . 

 This loop then j)asses through a series of metamorphoses, acquir- 

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



'^^riie mature exterior characters of the more prominent species 

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

 12, 12r(), one of the oldest species, retains many of the characters 

 of the earliest })eriod of the fourth growth-stage of Z. recurvirostra, 

 and therefore more nearly resendjles in form the primitive stock 

 which gave rise to Zi,gospira. In Z. saff'ordi (figs. 13, 13a) the 

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

 l)osterior third of the valves, and the shell is also more convex. 

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

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

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

 tioned. From Z. recurvirostra one line leads through Z. uphauii 

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

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

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

 defiecln., Z. niodcstft, Z. cincinncdiensis, and Z. kentuckyeiisis, the 

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

 conspicuous fold and sinus (jf Z. recurvirostra is gradually devel- 

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

 both lines are compared {Z. Jieadi and Z. kentuckyensis) very 



