Q'O CONTRIBUTIONS TO THE GEOLOGY OF MAINE. [bull.165. 
Silurian, and as the ancestors of the forms called again Camarotoechia 
in the Oriskany and later periods. 
In the Oriskany, with no Uncinulus, this group of shells is called 
Camarotoechia by Schuchert, although referred to still another group 
by Hall and Clarke (i. e., Plethorhyncha). 1 It becomes, therefore, 
of special interest to ascertain the meaning and taxonomic value of 
those characters which are used to differentiate these several groups, 
viz, Wilsonia, Camarotoechia, and Uncinulus. 
Wilsonia is said to have no cardinal process, but median septum and 
dental lamellae like Camarotoechia. 
Uncinulus is said to have a cardinal process, like Camarotoechia, and 
the median septum is described as present; but the slight division of 
the septum, which is described in Camarotoechia as forming a chamber 
between the teeth, is not distinctly seen in the typical Uncinulus. 
A critical analysis of the characters upon which the several divisions 
of the rhynchonellas here concerned are based develops the fact that 
the distinctions are formed on various degrees of development of three 
internal characters — (1) the cardinal process, (2) the dental lamellae, 
and (3) the median septum. 
The cardinal process is described as absent in Camarotoechia, Wil- 
sonia, and Rhynchonella; present and linear in Orthorhynchula, 
Khynchotrema, and Rhynchotreta; well developed in Uncinulus, and 
large in Eatonia. 
The dental lamellae are described as absent in Orthorhynchula, 
Rhynchotrema, Uncinulus (?), and Eatonia, and as present in Rhyn- 
chotreta, Camarotoechia, Wilsonia, and Rhynchonella. 
In the dorsal valve the median septum is described as thick in 
Rhynchotrema, prominent and divided in Rhynchotreta, prominent 
and with slight chamber in Camarotoechia, present or absent in Uncin- 
ulus, rudimentary in Hypothyris, short and bifid in Eatonia, and present 
in Rhynchonella. 
It will be noticed that those forms which are conspicuous in the 
Lower Helderberg are characterized by well-developed cardinal proc- 
ess, by absence of the dental lamellae, and by rudimentary development 
of the median septum. 
When this diagnosis is compared with the forms which are chiefly 
confined to formations earlier than the Lower Helderberg, it is to be 
noted that in regard to the first and third characters there is a definite 
departure from the earlier stage of development; in the second case 
there is no indication of sequence, so far as the definitions go. 
In both the first and second characters the forms called Camaro- 
toechia assume the characteristics of the earlier types. When this 
observation is associated with the well-known prominent development 
of the rhynchonellas, both in size and in numbers, in the Lower Hel- 
1 Pal. New York, Vol. VIII, Pt. II, p. 191. 
