244 ACROSALENIA. 



(fig. 2 c) ; the three uppermost pairs of plates have mere rudimentary tubercles ; the 

 sudden diminution in size between the fifth and sixth forms a remarkable feature in the 

 character of this species (fig. 2 a); at the base, the tubercles gradually diminish towards the 

 peristome (fig. 2 b). In young shells, and in some adult ones, the mihary zone consists of 

 two rows of granules ; but, in other varieties, it is formed of four rows, closely set together, 

 on the upper surface ; the plates are covered with numerous equal-sized granules, among 

 which the rudimentary tubercles are placed (fig. 2 a) ; the areolas of the three equatorial 

 tubercles are circular and confluent, the uppermost has a distinct scrobicular circle of 

 granules, but the others are confluent (fig. 2 d), and one row of granules separates the 

 areolas from the poriferous zones ; this description applies to the variety figured, but does 

 not agree with others. The Rev. Mr. Griesbach, on this point, has made the following 

 observations : " Among the very numerous specimens I found at Oundle, there is about an 

 equal number of two dissimilarly constructed individuals. I pointed this out to you 

 before (though you had seen it), and it is the same with the Wollaston specimens. When 

 I found that age would not account for the difference, I was set to thinking about sex, 

 making inquiries on this subject. I see from Professor Owen's lectures on the Invertebrate 

 Animals, that the ' sexes are in distinct individuals in the Echinoids as in star-fishes ;' 

 and Professor Forbes, in Decade III, ' Mem. of the Geol. Survey,' accounts for the 

 difference in form between some individuals of Micraster coranguinum, on the supposition 

 that it may possibly be due to sex. This was all I wanted, to confirm an impression I 

 had already entertained that the tall, ample form of this Acrosalenia pustulata, Forbes, 

 with its broad granulated space between the primary tubercles, is the female, and the 

 depressed one, with a narrow central area, the male. They are, without doubt, the same 

 species ; and I quite believe my sexual hypothesis to be true." As my excellent friend 

 has carefully examined a greater number of specimens of this urchin than any other 

 naturalist, it affords me much pleasure to record his explanation of a difficult problem, and 

 to add that it afibrds a physiological reason for an admitted fact. 



The apical disc is seldom preserved in this species (fig. 2 a); in one or two small 

 specimens the genital and ocular plates are m situ, but only a portion of the sur-anal 

 remains; the disc is pentagonal; the anterior and posterior pair of genital plates are 

 shield-shaped ; the right plate is the largest, and supports in its centre a spongy madrepori- 

 form body ; the single plate is crescentic, and its body is absent, to give place to the vent ; 

 the three anterior oculars are heart-shaped, and the two posterior pairs are elongated, and 

 form the sides of the anal aperture ; the sur-anal is composed of many pieces ; the three 

 anterior, however, are alone in situ — one central, and two latero-posterior plates, behind 

 these there was evidently a series to form the anterior boundary of the vent. The surface 

 of all the plates is covered with numerous granules, similar to those on other parts of the test. 



The base is flat ; the mouth opening is large, nearly, but not quite, one half the width 

 of the equatorial diameter ; the peristome is decagonal, with wide notches (fig. 2 b) ; the 

 ambulacral, however, being larger than the inter-ambulacral lobes. 



