12(5 Anxai.s of thk Cakne(;ie Museum. 



Inside the beak and umbo of the ventral valve is a thickening due 

 to the dei)osition of shelly matter while the cardinal area is being 

 added to. It is as wide as the area and striated in the same way. In 

 young specimens this thickening shows as a concave plate at the apex 

 of the delthyrium and might easily be mistaken for a deltidium. 



In later neanic and adult stages the ventral beak is incurved, but 

 does not project beyond that of the dorsal valve. The dorsal area 

 remains nearly as wide as that of the opposite valve, but instead of 

 lying at an angle as in the early neanic stages it is in the plane of 

 union of the valves. 



After the early neanic stages the teeth and crura are supported by 

 lamellce. The cardinal process is large and pyramidal, with the two 

 posterior faces roughened for the attachment of muscles. The crura 

 are long and slender, projecting outward and upward into the oppo- 

 site valve. At the distal ends they are flattened vertically. 



Comparison with Other Species. 



The ontogeny of Rliipodomella hybrida of the Niagara has been 

 worked out by Beecher and Clarke. {Memoirs N. Y. State Museum, 

 Vol. I, No. I, 1889, p. 17, pi. I, figs. 13-18.) The two species agree 

 closely in their developmental stages, one species differing only in the 

 earlier assumption of the characters of each stage. 



A shell oi R. hybrida .5 mm. in length and .75 mm. in width has 

 six striae on the dorsal valve and five on the ventral, while a specimen 

 of R. vanuA'emi .42 mm. long and .61 mm. wide has eight on the 

 dorsal and seven on the ventral. 



Tropidoleptus carinatus Conrad. (Plate VIII.) 



Hall, Pal. N. Y., IV, 1867, p. 407, pi. 62, fig. 2, 3. 



This well known species is very abundant in the silicified material, 

 and a collection of about seven hundred well-preserved specimens was 

 obtained after hundreds of fragmentary shells had been discarded. 



The smallest shell is 1.66 mm. in length and 1.46 mm. in width, 

 the largest, 22 mm. long and 28 mm. wide. 



There are many shells showing all gradations between the two. 

 Nearly all the small shells, and many of the adults, retain both valves 

 in their proper relations. 



The smallest shell figured by Hall is 4.6 mm. long and 4 mm. wide. 

 The largest is 31 mm. long and 40.4 mm. wide. 



