KONGL. ÖV. VET. AKADEMIENS HANDL. BAND. 19. N:0 7. 93 



versely, occupies tlic whole breadth of tlie intermdium, assuiniiig' u trapezoidal form, 

 and fully attains the ainbulacruui V, to whicli it becoiiies largely contiguous. By this 

 means the a 2 is pushed back, widely out of contact with the labrum, but retains, 

 like the two or thrce pairs of ventral plates, the triangulär, pointed shape and the 

 very oblique alternation, while the pre-anal, abdoiiiinal and dorsal plates, in propor- 

 tion as they are distant aborally, and become shorter and broader, hold a less oblique 

 position. 



The large sternum of the higher Amphisternous Spatangidse so universally con- 

 sists of two equal symmetrical halves in regular juxtaposition, as to sceni hardly to 

 give room for a query vvhether it niay not, after all, owe its form to a direct modih- 

 cation of that of the Meridosterni. However, it will seem to me, — notwithstanding 

 the incompleteness of my materials, — that there really are indications of such a 

 possibility, of its having originated through a gradual transposition of the obliquely 

 placed a 2 and b 2 oi the Meridosterni. The Adete Echinospatagus no doubt is rightly 

 numbered among the Amphisternous Spatangida^ but the plate a. 2 of its sternum, in- 

 ferior in size, triangulär and asymmetrical, hängs behind the b plate, as if retarded 

 in its growth, and anteriorly would not attain the labrum, did not this come to meet 

 it with a lateral prolongation of its aboral margin. It may be allowable, from the 

 whole of this singular feature to look back for the existence of earlier forms, perhaps 

 undiscovered yet, still more evidently transitional, showing how the a 2, moving for- 

 ward, first began to interpose itself between the b 2 and the ambulacruin V, at the 

 same time exchanging its transversely cuneate shape for one more fitting its work and 

 the place it was striving to occupy. If so, the two halves of the sternum of the higher 

 Spatangida; ought not to have been formed simultaneously, but in such a way that 

 the b 2 alone had first been transformed into the future right plate, and, after that, 

 the a 2 into the left one. And this supposition appears to acquire some degree of 

 probability, when, leaving Echinospatagus behind, we look forward at the modifications 

 displayed by the SpatangidaB of låter appearance and higher order, modifications all of 

 which are continuations of what has been observed in those of a lower. Thus among 

 the Prymnadetes, some genera, as Pala^ostoma ^), Hemiaster ^), Agassizia, Schizaster, 

 are seen to exhibit what may be regarded as marks of this movement, in the a 2 

 being behind and attaining adorally with a narrow point only the ineeting labrum. 

 Among the Prymnodesmian forms the earliest, Micraster ^), still shows these traces of 

 the asymmetry, but through the Avhole series of the higher, Tertiary and recent ge- 

 nera ''), the symmetry and the exact juxtaposition oi a 2 and b 2, of the episternals 

 a 3 and b 3 the pre-anals a 4, b 4, and even of one or other of the anal pairs of 

 plates, Q.S a 5, b 5, is thoroughly established: their sutures have become rectilinear 

 by the disappearance of the posterior inner truncation, and tlie angular iniddle suture 

 is confined solely to the still alteriiating rows of abdominal and dorsal plates. 



1) Etudes pl. XXXII. 



2) Ib. pl. XXVI, XXX, XXXI. 



3) Ib. pl. XXXIII. 



') Ib. pl. XXXIV— XUI. 



