70 BRISSOPSIS LYRIFERA. 



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 

 specimens from off the mouth of the Mississippi, the subanal fasciole was in 

 some cases well defined, in others somewhat indistinct; in others again only 

 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 

 differs 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 extremity 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, tin." 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 ambulacra! plates alone is bare, the posterior row being closely 

 crowded with miliaries. The Hat surface enclosed within the branch of (he 

 anal lasciole is hare in the central region, somewhat coarsely tuberculated 

 at first towards the exterior; gradually the 1 uherculation becomes smaller, 

 until it passes into the fine miliaries which form the anal branch of the 

 lasciole. extending from the subanal to the peripetalous fasciole along the 

 central line of the posterior row of ainhulacral plates. 



