61G MOIf.A STYGIA. 



gate apical system, its steep sloping anterior extremity. On the actinal side 

 the actinal plastron is rounded posteriorly, projecting into a sharp beak. 

 The sunken ambulacra are narrow; the edges slope suddenly, and are not 

 gradually curved in, as in the other species of the genus. The posterior am- 

 bulacra are straight, proportionally larger ; the course of the anterior pair 

 forms but a slight angle near the apex ; the posterior edge, which projects 

 over the lateral anterior ambulacra in M. atropos, and which is more than 

 one third the length of the whole ambulacral depression, is, in this species, 

 scarcely one fifth of the length of the corresponding petal. In the odd an- 

 terior ambulacrum the triangular space dividing, in M. atropos. the ^loove 

 into two cavities, is here reduced to a very indistinct space (lip), forming 

 gradually sloping sides, slightly re-entering in the centre, extending to the 

 extremity of the petal, which terminates at a considerable distance above the 

 ambitus, the ambulacral groove disappears very gradually, becoming a slight 

 indentation at the ambitus, while it is quite deep, and extends to the actino- 

 stonie, in M. atropos. 



The outline of the test, as seen from the actinal side, is broadly rounded 

 anteriorly, pointed posteriorly. The actinostome is remarkably large, nearly 

 twice as broad, in proportion to its length, as in M. atropos. The actinal 

 plastron is elongate, rounded towards the posterior extremity, which termi- 

 nates in a sharp keel. It is covered by a uniform t uberculation of large tu- 

 bercles, scarcely becoming smaller at the posterior end : while, in M. atropos. 

 the diminution in the size of the tubercles is quite rapid and marked, as the 

 distance from the actinostome increases. The anal system is very elongated, 

 narrow, elliptical ; but the general arrangement of the plates is the same. — an 

 outer row of large plates, of nearly uniform size, surrounding a circular area 

 of smaller irregular plates in which the anal opening is placed. The course 

 of the lateral fasciole is nearly horizontal, forming a sharp curve as it passes 

 towards the anal system to become the anal fasciole ; it extends far below the 

 anal system to the edge of the test, to a short distance above the beak of the 

 actinal plastron. In the specimens examined the tuberculation of the abac- 

 tinal surface is somewhat more crowded and proportionally smaller than in 

 M. atropos. A comparison of the measurements of this sf>ecies with those of 

 M. atropos will show at a glance the great differences between them. 



Dist. Length Dist. Length Iliitrlit 



Long. Tmns. Apical Syst. Ant. Ijit. of Ant. Amb. Poet AnsiSyst. 



Diam. [Ham Iloight. Ant. Edge. Amb. l'et. from Apes. Amb. Pet. above Edge* 



49.5 43. 40. 28. 28. 4. 17. 16. 



Red Sea ; Zanzibar. 



