70 BRISSOPSIS LYRTFERA. 
branch is clear and well marked, in others it becomes gradually reduced and 
indistinct, or finally disappears entirely. The subanal fasciole also appears 
in the American specimens to be subject to great variations. In the globular 
sjDecimens from off the mouth of the Mississippi, the subanal fasciole was in 
some cases well defined, in others somewhat indistinct; in others again onl}^ 
very indistinct and disconnected parts could be traced ; and finally in others 
it had disappeared entirely, as is the case in the genus Toxobrissus, which 
diflers from Brissopsis only in having no subanal fasciole and confluent lat- 
eral ambulacra, characters which are here distinctly shown to occur in this 
species of Brissopsis in specimens found in different localities. The speci- 
mens with globular test have retained that embryonic feature alone, while 
the petals and fascioles have developed in what we might call the normal 
manner. The excessively elongate and flattened forms found off Jamaica 
retain of the embryological characters the confluent ambulacra and the 
position of the anal system on the anteriorly sloped surface, a feature re- 
calling the time when the anal system was distinctly placed on the abactinal 
surface of the test, and not on the vertically truncated posterior extremitj^ as 
is usually the case. 
In Brissopsis, as in Macropneustes and other Spatangoids, the centrali- 
zation of the tubercles on certain parts of the plates shows how closely this 
is connected, on the one hand, with the formation of V-shaped fascioles in the 
interambulacral and ambulacral areas, as in Macropneustes. In the inter- 
ambulacral areas the absence of tubercles leaves bare the median and hori- 
zontal sutural lines, the median spaces and adjoining angular connections 
being broader than the horizontal lines. The anterior ambulacra are bare 
from the extremity of the petals, while in the posterior ambulacra the an- 
terior row of ambulacral plates alone is bare, the posterior row being closely 
crowded with miliaries. The flat surface enclosed within the branch of the 
anal fasciole is bare in the central region, somewhat coarsely tuberculated 
at first towards the exterior ; gradually the tuberculation becomes smaller, 
until it passes into the fine miliaries which form the anal branch of the 
fasciole, extending from the subanal to the peripetalous fasciole along the 
central line of the posterior row of ambulacral plates. 
