ON THE EMBRYOLOGY OF ECHINODERMS. 17 



ters. The Cidaridse make their appearance first, and the other genera, of vhich a 

 few representatives are found, belong to a family closely allied to the Cidarida?, the 

 Hemicidaridaj, a prophetic and synthetic family combining features of the Cidarida;, 

 which have preceded them, and of the Pseudo-Diadcmatidtc, which are to follow them. 

 We have not a single Clypeastroid, Echinolamp, or Spatangoid. During the Lias the 

 Clypeastroids make their appearance, but the type which represents the suborder, the 

 genus Collyrites, is not a Clypeastroid, as we understand them in the present epoch. It 

 is a type which is the forerunner of the true Clypeastroids, and which apes the char- 

 acters of the Spatangoids. The Echinoids of the Lias have not the strong, embryonic 

 character which belonged to the Trias; we find such genera as Hemipcdina, Diadeniopsis, 

 and the like, which remind us forcibly of the Diadematidee of our present time. In the 

 lower stages of the Jurassic period the Echinoids are of still more varied genera, — such 

 as Holectypus and Pygaster, — being forms which recall to us the embryonic stages of 

 our Clypeastroids. The Echinolamps are likewise introduced with Pygurus and Echi- 

 nobrissus ; while it is only in the lowest Cretaceous deposits that the first Spatangoids 

 appear as Holaster and Toxaster. If we compare the appearance of the Clypeastroids 

 in geological ages with the embryonic stages of which I have given a short account, 

 we shall find that the first true Clypeastroids (Clypeastroids such as we know them in 

 our own time) are such forms as Scutellina and Lenita, in the Eocene ; it is only in the 

 Myocene that the genus Clypeaster appears and takes its greatest development, accom- 

 panied with a large number of true Scutelloe ; and it is not till the Pliocene that genera 

 resembling Amphiope, Encope, and finally Mellita, make their appearance, showing a 

 closeness of agreement between the order of development and the geological succession 

 carried out to the fullest possible extent. 



OPIIIURANS. 



To the extensive investigations of Miiller about the embryology of Ophiurans and 

 Holothurians I have but little to add, and I consider myself very fortunate to have 

 been able to increase our knowledge of two of the orders of Echinodcrms after what 

 has been done by him. 



OphiophoUs hellis Lym. 



Two Ophiurans are quite common at Nahant ; one, Amj)hmra squamata Sars ; 

 the other, OphiophoUs hellis Lym. The latter, unfortunately, does not lay eggs from 

 which the plutean stage is developed, as the young Ophiurans are never nomadic. 

 The eggs are laid in bunches, from which, according to the observations of Professor 

 Agassiz, made as far back as 1849, the young OphiophoUs is developed, very much 



VOL. IX. 3 



