No. 7.] AMERICAN ASSOCIATION. 393 



produced the genera in whicli these modifications actually take 

 place. But, starting from Pygaster, we naturally pass to Holec- 

 typus, to Discoidea, to Conoclypus, on the one side, while on the 

 other, from Holectypus to Echinocyamus, Sismondia, Fibularia, 

 and Mortonia, we have the natural sequence of the characters of 

 the existing Echinanthidae, Laganidae, and Scutellidae, the greater 

 number of which are characteristic of the present epoch. If we 

 were to take in turn the changes undergone in the arrangement 

 of the plates of the test, as we pass from Pygaster to Holec- 

 typus, to Echinocyamus, and the Echinanthidae, we should have 

 in the genera whicli follow each other in the paleontological record 

 an unbroken series showing exactly what these modifications have 

 been. In the same way, the modifications of the abactinal and 

 anal systems, and those of the poriferous zone, can equally well 

 be followed to Echinocyamus, and thence to the Clypeastridae ; 

 while a similar sequence in the modifications of these structural 

 features can be followed from Mortonia to the Scutellidae of the 

 present period." 



Passing next to the embryological development of the several 

 families, he remarked : " Among the Clypeastroids the changes 

 of form they undergo during growth are most instructive. We 

 have in the young Fibularinae an ovoid test, a small number of 

 <}oronal plates surmounted by few and large primary tubercles, 

 supporting proportionally equally large primary radioles, simple 

 rectilinear poriferous zones, no petaloid ambulacra, — in fact, 

 scarcely one of the features we are accustomed to associate with 

 the Clypeastroids is as yet prominently developed. But rapidly 

 with increasing size, the number of primary tubercles increases, 

 the spines lose their disproportionate size, the pores of the abac- 

 tinal region become crowded elongate, and a rudimentary petal is 

 formed. The test becomes more flattened, the coronal plates in- 

 crease in number, and it would be impossible to recognize in the 

 young Echinocyamus, for instance, the adult of the Cidaris-like or 

 Echinometra-like stages of the Sea-urchin, had we not traced them 

 step by step. Most interesting, also, is it to follow the migrations 

 of the anal system which, to a certain extent, may be said to re- 

 tain the embryonic features of the early stages of all Echinoderm 

 embryos, in being placed in more or less close proximity to the 

 actinostome. What has taken place in the growth of the young 

 Echinocyamus is practically repeated for all famillies of Cly- 

 peastroids; a young Echinarachnius, or Melita, or Encope, or a 



