KONGL. SV. VET. AKADEMIENS HANDL. BAN]). 19. M:i) 7. 93 



versely, occupies the wliolc breadtli of tlie interradiuni, assuiuiiiy a trapezoidal form, 

 and fully attains the ambulacrum V, to which it becomes largely contiguous. By this 

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

 like the two or three pairs of ventral plates, tlie triangular, pointed shape and the 

 very oblique alternation, while the [)re-anal, abdominal 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 Spatangida) so universally con- 

 sists of two equal symmetrical halves in i-egular juxtaposition, as to seem hardly to 

 give room for a query whether it may not, after all, owe its form to a direct modifi- 

 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 of the Meridosterni. The Adete Echinospatagus no doubt is rightly 

 numbered among the .4mphisternous Spatangid*, but the plate a 2 of its sternum, in- 

 ferior in size, triangular and asymmetrical, hangs behind the b plate, as if retarded 

 in its growth, and anteriorly Avould 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 h 2 and the ambulacrum 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 Spatangidai of later 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 Palaiostoma '), Hemiaster ^), Agassizia, Schizaster, 

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

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

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

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

 nera 0, the symmetry and the exact juxtaposition of a 2 and b 2, of the episternals 

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

 plates, as a 5, 6 5, is thoroughly established: their sutures have become rectilinear 

 by the disappearance of the posterior inner truncation, and the angular middle suture 

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



») Etudes pi. XXXII. 



2) lb. pi. XXVI, XXX, XXXI. 



3) lb. pi. XXXIII. 



*) lb. pi. XXXIV— XUl. 



