BROWN, RUGOSE CORALS 
91 
another step to the genus Enterolasma and seems to be represented by the 
single species Enterolasma caliculum, occurring in the limestones and shales 
from the Clinton to the Cobleskill. In its younger stages Enterolasma cali¬ 
culum passes through the same development as S. profundum and S. corni- 
culum. It does not appear to be a derivative of S. corniculum, however, 
but seems to have been derived from another line by parallel development, 
because this form never attains the large size or very numerous and thick 
septa of S. corniculum. Yet the septa do become more or less radially 
arranged, and the acceleration of the counter quadrants over the cardinal 
is carried even farther than in S. corniculum. It is even seen in this 
species that the acceleration of the counter quadrants over the cardinal 
quadrants progressively increases during the life of the individual and be¬ 
comes strongly marked during the later stages. 
While the fully developed septa of S. corniculum project freely to the 
center and do not unite to form a pseudocolumella, the septa of this species 
do unite at the center to form a somewhat incomplete or irregular pseudo¬ 
columella, which is the characteristic of the next step in advance along the 
line of morphological development. Throughout the Siluric, although 
individuals are very numerous, there is no increase or marked variation of 
the species. The ancestral stages of development are crowded far forward 
into the early part of the individual development, and the later stages 
are chiefly characterized by the peculiar, incompletely developed pseudo¬ 
columella. 
It is probable that [ Zaphrentis ] racinensis from the Niagara beds of Wis¬ 
consin represents one of the first lateral branches from the main Streptelasma 
stem, but material of this species is not available for ontogenetic study. 
In the lower Devonic beds, as represented by the limestones of the 
Lower Helderberg series, Enterolasma is still the dominant genus and is 
here represented by the species E. strictum. In this species, although the 
pseudocolumella is far from being solid, it is yet much more substantial 
than in the Siluric species. Another characteristic which marks a change 
between the Siluric forms and this is the manner of attachment of the inner 
margins of the tertiary septa to the adjacent secondary septa. In all the 
earlier forms, the tertiary septa are free throughout their length. In this and 
the majority of the later forms, they are attached by their inner margins to 
the adjacent primary or secondary septum immediately dorsal to them. 
During about one half or even more than one half of the geological time 
through which the Streptelasma group of rugose corals is distributed, its 
range of variation is very limited, and its changes are along conservative 
lines. With the close of the lower Devonic, however, this limited variation 
and conservative development is suddenly ended, and the Streptelasma 
