No. 1596. DALMANELLAS OF THE CHEMUNG—WILLIAMS. 45 
Size.—The collections at present in hand show a geographical dif- 
ference in expression of size and a definite law of increase in size on 
continued occupation of the area. 
The species Orthis (Dalmanella) leonensis Hall is the common 
form in western New York, and so far as known there are no Dalma- 
nellas in the western New York Chemung attaining a diameter of 20 
mm., and rarely is a specimen seen exceeding 15 mm. in width. 
Further search may modify this statement. 
The collections from central New York east of the Genesee River 
show another law. The Dalmanellas from the geological sections of 
Watkins Glen and Catatonk quadrangles exhibit a general increase 
in size with each successive stage from their first appearance to their 
final exhibition. The first zone, called the Dalmanella danbyi zone, 
at the base of the Cayuta member, about 150 feet thick, contaims only 
small individuals, the majority of which do not exceed 12 mm. in 
width and the largest of which probably do not exceed 15 mm. The 
specimens in the succeeding fossiliferous zones of the Cayuta member 
are rarely under 20 mm. in width and an occasional specimen reaches 
a width of 40 mm. In the separate fossiliferous zones of the middle 
and upper Cayuta the average size differs, but for each zone the 
variation in size is restricted within narrower limits than the ex- 
iremes, showing that local conditions of environment exercised some 
control upon the dimensions. In some zones the average size is about 
2() mm., in other zones about 25 mm. In the zones with the smaller 
average there is evidence of slower growth, while those of larger 
average give indication of luxuriance of growth. In general, how- 
ever, the zones near the top of the Cayuta member exhibit larger 
dimensigns than those of the earlier zones of the Cayuta member. 
The specimens from the highest’ zones of the Cayuta member and 
the later representatives of the genus show the greatest average d1- 
mensions, about 30 mm., running up to 40 mm. or over in width. 
Contour forms:of shells of Dalmanella—A comparative study of 
the Dalmanellas has demonstrated the importance of designating by 
some distinct term the form of contour or outline of the shell without 
regard to size or taxonomic rank assigned to the specimens. This 
matter of contour seems to be an expression of differential rate of 
growth of the shells in various directions during growth from 
embryonic to adult age. A shell in which the growth proceeds at a 
relatively uniform rate in all directions will assume a circular con- 
tour. ‘The shells of Dalmanella grow much more rapidly toward the 
front than toward the cardinal side of the umbones or starting point 
of growth. In general the contour of a Dalmanella is deltoid, and 
looked at from a point vertical to the hinge line the cardinal margin 
presents a straight line. The contour rapidly broadens from the end 
of the straight part of the hinge so that the shell assumes in general 
