TRIPYLUS EXCAVATUS. 589 



way along the rounded keel, extending between the apical system and the 

 truncated anal extremity, which is nearly level, only slightly arched. An- 

 terior extremity sloping gradually at first, then arching towards the ambitus. 

 Anterior groove narrow, extending to the actinostome. Lateral ambulacra 

 broad, deeply sunken, the anterior rectangular, extending nearly to the 

 ambitus, rounded at extremity ; the posterior shorter than the anterior, with 

 posterior edge more arched than anterior edge. Apical system flat, with 

 three huge elliptical genital openings. The peripetalous fasciole is rounded 

 anteriorly, where it descends near to the ambitus, re-entering slightly near 

 the extremity of the anterior lateral petals, forming a deep re-entering 

 angle in the lateral posterior interambulacral areas, and only a slight in- 

 dentation across the odd posterior interambulacrum ; the posterior part of 

 the fasciole is narrow from the point of branching of the lateral fasciole (the 

 extremity of the anterior petals) ; this arches towards the apical system in 

 the median interambulacral space, curving gradually downward, and passing 

 below the anal system near the ambitus, forming a nearly straight line 

 across the posterior edge. The anal system is situated at the upper end 

 of the posterior extremity ; it is longitudinally elliptical, small, strengthened 

 by an outer row of larger plates, gradually becoming smaller towards the 

 anal opening. The actinal side is flat. The posterior ambulacra form dis- 

 tinct, bare avenues, extending nearly to the ambitus in both sides of the 

 broadly triangular actinal plastron. The actinostome is narrow, transverse, 

 with a prominent posterior lip, nearly one quarter the width of the test in 

 length. The tubercles of the actinal side are large near the ambulacra, 

 diminishing rapidly towards the ambitus ; the whole abactinal part of the 

 test is covered by a close, minute granulation, carrying short, club-shaped 

 spines, slightly curved, grooved at the top, and flattened. On the actinal 

 edge of the sunken ambulacra are found larger tubercles, which carry 

 similar, only larger, spines, extending from both sides across the ambulacra, 

 and hiding them in part. The spines of dry specimens were of a light- 

 green color ; the denuded test grayish-pink. 



