2,6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 149 



Order Spatangoida Claus 



Family BRISSIDAE Gray 



Genus PLAGIOBRISSUS Pomel 

 PLAGIOBPJSSTJS GRANDIS (Gmelin) 

 Plate 8, figures 1-6; plate 15, figures 2, 3 



This species is characterized by its large low test, often more than 

 200 mm. long, tan color, and extremely long spines which curve back 

 over the upper surface. The bare test is distinguished by the large 

 tubercles on the upper surface, the four slightly depressed curving 

 petals of unequal length (anterior shorter) the narrow plastron, and 

 the thinness of the plates of the test. It differs from the other large 

 spatangoid, Meoma ventricosa, with which it is often associated, by 

 its lower, lighter test, long spines and large tubercles on the upper 

 surface, lighter color, and narrower plastron. 



Juvenile morphology. — A denuded specimen only 35 mm. long (pi. 

 15, fig. 2) was collected at locality 30. This specimen differs from 

 an adult in many of the same ways that young Meoma ventricosa 

 (described below) differs as a juvenile. It is relatively higher and 

 less angular than the adult, its petals are less depressed, and the peri- 

 proct is not visible adorally. The posterior petals are much shorter 

 proportionately, extending only half the distance from the apical sys- 

 tem to the margin. This contrasts to the proportionately long petals in 

 adults which extend more than two-thirds that distance. The anterior 

 petals are more divergent, and all petals are relatively wider and 

 straighter in the young specimen. The peristome is larger relative 

 to the size of the test, and the labrum less strongly developed. Un- 

 fortunately, no spines are preserved on this small specimen, so it is 

 impossible to determine the relative change in their length with growth 

 of the test. Genital pores are present in this small specimen, sug- 

 gesting that P. grandis reaches sexual maturity at a smaller size than 

 M. ventricosa. 



Occurrence. — Plagiobrissus grandis is found in sandy areas where 

 grass and algae are sparse or absent, associated with other sand 

 dwellers such as Meoma ventricosa, Clypeaster subdepressus, Encope 

 michelini and Lcodia sexiesperforata. Specimens of this species are 

 relatively rare (or at least, infrequently found) so their habitat could 

 not be determined accurately ; they seemed to be most common at the 

 edges of the grassless areas, 



The species was found only well offshore, from the White Bank 



