182 SYNOPTIC COLLECTION. 



tals and the two oculars form the ring around the anal 

 disc, the other three oculars being crowded outside the 

 ring. 



The globular form and melon-like aspect of Echinus 

 melo Lam. (No. 334), are striking. These melon-like sec- 

 tions mark off distinctly the five ambulacral and five inter- 

 ambulacra] areas. 



The plates of the anal area are numerous, small, and 

 irregular, while the five ocular plates are all crowded out- 

 side of the ring. 



The spines of Echinus acutus Lam. (No. 335), are far 

 removed from those of primitive and embryonic forms, 

 being small, short, and similar on both ambulacra and in- 

 terambulacra. In this specimen the tube feet are seen 

 extending from the shell. 



IRREGULAR SEA URCHINS. CLYPEASTROIDS. 



Pygaster patelliformis Ag. (No. 336), is one of the 

 primitive Clypeastroids. The corona is dome-shaped 

 above and flattened below, resembling the regular sea 

 urchins. The mouth is placed near the center of the 

 oral area, but the anus is not directly opposite as in the 

 sea urchins so far examined. It extends from near the 

 apical disc to the margin and is large in size. The ambu- 

 lacral areas are narrow with simple vertical rows of paired 

 pores, while the interambulacra are broad. 



Young Clypeastroids in general possess a small number 

 of plates in the globular corona, a few large spines and 

 tubercles, simple vertical rows of pores with no petal-like 

 pattern on the dorsal side. Internally there are no parti- 

 tions. With age the corona becomes more flattened, the 

 number of plates increases, the spines grow smaller, and 

 the pores form into petals, proving that the petaloid con- 

 dition is a specialized one ; and that the sea urchins pos- 

 sessing it should be placed after those whose pores are in 



