CLYPEASTER SUBDEPRESSUS. 309 



the edge of the test of these young specimens a striking appearance (PI. 

 XIII. f. 14). We find frequently on the edge of the test of small specimens 

 the glassy tubercles which are so characteristic of Echinoneus, but they do 

 not appear to be constantly present, and furnish no additional clew as to 

 their function. The great development which the pores of the ambulacral 

 furrows take on the upper part of the test in small specimens is shown in 

 PL XIII. f. 12 ; in young specimens the pores of the furrows are limited 

 to the narrow line of the vertical and horizontal sutures of the plates, both 

 on the lower and upper part of the test. 



Littoral, — 34 fathoms. 



The development of flat Clypeastroids of the type of Clyp. subdepressus is 

 most instructive, tending to show that, in connection with the development 

 of the Scutellidae hereafter described, we must probably introduce a complete 

 reform among the genera recognized as Lenita, Scutellina, Runa, and other 

 minute Echinoids, which may eventually prove to be nothing but the young 

 of other Clypeastroids, such as Mellita, Scutella, Laganum, Clypeaster, En- 

 cope, and the like ; but the want of sufficient material prevents me from enter- 

 ing into this comparison more in detail. Though Ave know now, from what I 

 shall show below, that the Scutellidae pass through phases which cannot be 

 distinguished from Moulinsia, Fibularia, Runa, Scutellina, and the Clypeas- 

 troids proper pass through a stage' of growth similar to Echinocyamus. For 

 similar reasons I am inclined to consider Fibularia as the early stage of some 

 Clypeastroid. The absence of partitions in some species, I think, can easily 

 be accounted for, as they are developed only later. We have a species of 

 Fibularia from the Sandwich Islands, in which there are no partitions when 

 very small, while in the adult these partitions are most rudimentary. Greater 

 material than I possess is necessary to elucidate the affinity of the genus, 

 which certainly has all the features of immature Clypeastroids. 



Among the Clypeastroids, as well as among the regular Echini and Spa- 

 tangoids, a large number of fossil genera have been discriminated which are 

 based upon characters of no permanent value ; differences which are not even 

 specific among recent Echini, where we have had an opportunity of examin- 

 ing large series of various ages, having been so exaggerated as to become 



