382 Prof. S. Loven on the Structure of the Echinoidea. 



form. A different condition stands in connexion with this. 

 Most of the living genera of Spatangidse have an infraanal 

 fasciola, which forms below the periproctium an oval ring, 

 within which, as Johannes Mliller first observed in Brissojysis 

 lyriferaj long tentacular cirri come forth, the pores of which, 

 therefore, are also situated within its circumference. At the 

 same time the case is that, in all the genera furnished with 

 an infraanal fasciola, in both the inner rows of the bivium the 

 sixth plate and some of the following ones — namely, two in 

 Palceotrojms^ SpatanguSj and Meoma (which has an incomplete 

 fasciola), three in Maretia^ Echinocardium^ Lovenia^ Brtssojjsis, 

 and Eu^atajuSj four in Bn'ssxis, Kleinia^ Plagionotus^ and Xan- 

 thobrissus, and even six in Breynia — have a different form 

 from the others, inasmuch as thej are drawn out towards the 

 middle line of the test, and form together a produced wedge. 

 In the seventh and following of these plates, moreover, the 

 tentacular pore has so shifted that it comes within the fasciola. 

 In all these genera it is also the rule that the first six plates of 

 the inner rows of the bivium correspond with the outer mar- 

 gins of the labrum, sternum, and episternum, the three ventral 

 parts of the unpaired interradium, and that the episternal pair 

 of plates forms, with the nearest pair of abdominal plates on 

 both sides, an angle {angulus ejnsternalis) which receives this 

 wedge of produced plates, and in different genera is more or 

 less deep or open. A young Brissopsts lyrifera, 4*6 millims. 

 in length, presents in these respects the same characters as the 

 full-grown individuals j in both it is the fifth ambulacral 

 plate that corresponds to the angle between the sternum and 

 episternum, and the sixth to and with the ninth that enter the 

 episternal angle ; and hence it is clear that in this part of the 

 bivium during growth no shifting takes place in these plates 

 in the direction of the peristome 5 but we rather see the ambu- 

 lacral plates here, except those of the peristome, become 

 somewhat elongated with age. In the genera which are 

 destitute of a fasciola infraanalis (such as Schizaster fragilis^ 

 Desoria, Agassizia^ Abatus, Atrapus, and Hemiaster) the 

 plates corresponding to the scarcely perceptible angidus epi- 

 sternah's, which is not always alike on both sides, are but 

 little or not at all produced in a direction towards the middle 

 line of the test, but have almost completely the form of the pre- 

 ceding ; and the number of plates of the bivium which occupy 

 the same length as the three ventral parts of the unpaired 

 interradium is indeterminate, — in Schizaster seven, in Afrajms 

 six, in Ahatus seven and a half on the left side and six and a 

 half on the right, in Desoria eight and a half, in Agassizia 

 six and a half. This irregularity is most considerable in 



