l8o NEW YORK STATE MUSEUM 



It is interesting to note that Girty finds this species, or one prac- 

 tically inseparable from it, in the Moorefield shales of Arkansas. 



Composita dawsoni (Hall & Clarke) 



Athyris subtilita Davidson. Quart. Journ. Geol. Soc. Lond. 19, 

 p. 170, pi. 9, fig. 4, 5, (not A. subtilita Hall) 1863. Quoted by Dawson, 

 Acad. Geol., p. 290, fig, 88a-c, 1868. 



Seminula dawsoni Hall. 13th Ann. Rept. N. Y. State Geol., 

 p. 652, pi. 47, fig. 32-34, 1894; 14th Rept., p. 359, pi. 9, fig. 14-16, 1897. 



The specimens referred to this species are not very abundant, 

 are distorted and poorly preserved. It can not be stated that 

 they certainly belong to the species to which they are here referred, 

 though there appears to be little doubt of it. 



Nucula iowensis White & Whitfield var. magdalenensis nov. 

 Shell minute, triangular in outline, very ventricose. Beaks nearly 

 terminal posteriorly, little elevated ; dorsal border slightly arcuate, 

 sloping forward to the pointed anterior end which rounds abruptly 



into the nearly straight but gently con- 

 vex ventral margin making an abrupt 

 turn upwards at the posterior extremity. 

 Nucula iowensis var. magda- Postcrior margin truucatcd from beaks 



lenensis nov. Cardinal view x 5. , . p. . , , 



Surface of left valve X 4 to vcntral extremity, buriace marked 



by regular concentric crenulated striae separated by depressions of 

 about equal width. 



Dimensions. Length, about 4 mm ; height, 2.2 mm ; convexity, 

 2 mm. 



Remarks. Winchell's description of N. iowensis is followed 

 by these remarks : 



"The shell appears to be subject to considerable variation at dif- 

 ferent stages of growth ; young specimens often being distinctly 

 triangular, with the posterior end very sjjort, and the basal margin 

 but little arched, while the old specimens are subovate and the 

 posterior end more prolonged. This description of young indi- 

 viduals tallies very closely with the species in hand which may be a 

 variety of Winchell's species. All our specimens are minute. 



While resembling the description of the young of Winchell's 

 species, our specimens are very different from the adult forms. His 

 specimens are larger than the largest of ours. The dimensions above 

 given are for the largest specimen. 



Our specimens dififer from N. houghtoni Stevens in being 

 more elongate with straighter ventral margin, as they do from 

 N. parva McChesney. It is related to N. rectangula 



