l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I44 



crowded pores. There is considerable evolution within Pygunis, and 

 the genus is divided into three subgenera, the earlier, more primitive 

 Pygurus (Mepygurus), the later Pygurus {Pygunis), and Pygurus 

 (Pyguropsis). Pygurus (Pygurus) differs from Pygurus (Mepyg- 

 urus) in having shorter, broader phyllodes with fewer pore pairs, a 

 higher test, shorter petals not extending as far adorally, with narrower 

 poriferous zones, and large adoral tubercles. Pygurus (Pyguropsis) 

 differs from Pygurus (Pygurus) in having a higher test. Pseudo- 

 pygurus is descended from Pygurus and very closely related to it, 

 differing only in having a less developed petal III. Clypeus differs 

 from Pygurus in having a supramarginal instead of inframarginal 

 periproct, and is probably descended from the same stock that pro- 

 duced Pygurus. Astrolampas and Bothryopneustes are tentatively 

 placed in this family but appear intermediate between it and the 

 Nucleolitidae. Astrolampas, because of its low test and wide petals, is 

 allied with Pygurus, but the pores in its phyllodes are not crowded, 

 having only two series of pore pairs in each half -ambulacrum, a char- 

 acter common with the Nucleolitidae. In Bothryopneustes the petals 

 and the phyllodes are intermediate in development with those typical 

 in the Nucleolitidae and the Clypeidae. 



The Nucleolitidae, like the Clypeidae, are descended from the 

 Galeropygidae or a close ancestor of the Galeropygidae. They are 

 more advanced in having more pronounced petals and usually fewer 

 pore pairs in their phyllodes. All the genera in the family, with the 

 possible exception of Pscudosorella, appear to be derived from 

 Nucleolitcs. All their phyllodes are very similar in having two series 

 of pore pairs in each half -ambulacrum; most of them have similar, 

 open petals with narrow poriferous zones, and elongate tests. In 

 Pscudosorella the phyllodes are similar to those in Nucleolites, but 

 the test is very wide, a character found in Galeropygus. Of all the 

 genera in the family, Clypeopygus has the strongest affinities with 

 Nucleolites. Both genera have very similar petals, phyllodes, and 

 position of periproct, and differ only in that Clypeopygus has a more 

 depressed test. Phyllohrissus is closely related to Nucleolites, because 

 both genera have narrow, straight, open petals, and very similar 

 phyllodes, but it appears to be more advanced in having a more 

 elongate test, more developed bourrelets, and a more marginal peri- 

 proct. Phyllohrissus is very similar to Catopygus and probably closely 

 related, having similar petals, strikingly similar phyllodes with the 

 inner pore of each pair greatly reduced in size, and well-developed 

 bourrelets. Catopygus appears to be slightly more advanced with a 



