williams.] SECOND SERIES OF RHYNCHONELLAS. 75 
To generalize this theory of their growth, it may be said that the 
characteristic elements of plasticity, which at the Pentamerus stage 
were confined to early stages of individual development, became 
conspicuous and dominated the growth of relatively later and later 
stages of development with the passage of time. This accords with 
the general trend of hypothesis advanced by Hyatt. 
Another series of rhynchonellas is represented by three fairly well- 
differentiated species described or referred to by Billings under the 
names R. aspasia and " R. like hialveata" Hall; and another form 
referred to under the description of the former in likening the species 
R. aspasia to R. altiplicata and R. acutiplicata Hall, and called "old 
specimens." 
In the present collection the larger forms of this series described 
by Billings as R. astasia ("7 by 6 lines in size") are not in evidence, 
but one of the smaller forms shows the characters corresponding to 
the description given rather than to the figures on the plate. 
The two species in the present collection, with several well-marked 
specimens each, are referable to R. aspasia Billings and R. bialveata 
Hall. These belong, further, to a group of species of the Delthyris 
Shaly limestone in New York, including also the species R. altiplicata, 
R. transversa, R. rudis, and in the Lower Pentamerus the form evi- 
dently referred to by Hall as "R. semijMcata Conrad." Possibly R. 
formosa and R. sulciplicata Hall also belong to the series. 
The specimens from the Maine fauna here referred to R. aspasia 
Billings (and, judging from the descriptions, the series of species 
described from New York by Hall may be included) are marked by 
their triangular form, with rather distinct narrow beaks, straight sides, 
few angular plications, a well-developed fold, and sinus at the front of 
the shell reaching halfway to beak. The shells are, in general, rather 
thin, and more or less flat along the middle part of the surface. 
The description of R. aspasia by Billings seems to cover two forms, 
while the figure * in the descriptive paper applies to the typical form 
only. The following statement is referred to: "In old specimens, 
about twelve ribs on each valve, of which there are two or three in the 
sinus and three or four in the fold," and later the less angular rounded 
form of the sides of "old specimens." 
The association of these "old" and larger specimens with the 
smaller typical forms brings in the characters which by Hall are made 
to distinguish other species, such as R. rudis, R. transversa, and 
apparently some of those referred to R. formosa. on the plates. Resem- 
blance to R* acutiplicata Hall is evident in the larger of the speci- 
mens in the collection. 
Billings's figure well represents the typical characteristics of the 
larger of the specimens referred to this species; another of our speci- 
i PI. Ill, fig. 6. 
