Yol. 51.] ANNIVEESAEY ADDEESS OE THE PEESIDENT. lxxxiii 



apart as two distinct classes. There does not appear to me, 

 however, to he sufficient reason given for arbitrarily separating 

 these orders from the class Cetjstacea, of which, in Palseozoic times, 

 they certainly formed a perfectly natural and homogeneous part. 



I am quite willing to adopt the view that the trilobita are 

 ancestrally connected with Limulus ; that Limulus may be related 

 through Hemiaspis with Eurypterus ; but all the intermediate forms 

 have not yet been met with. That some ancestral Eurypterid must 

 have given rise to Scorpio cannot, I think, be doubted; but it 

 must have been in pre-Silurian times, for Peach and Lindstrom's 

 Palceophonus had already appeared in the Upper Silurian of Lanark- 

 shire and Gotland as a terrestrial pulmonated form, while a similar 

 land-scorpion had been discovered by "Whitfield in the Silurian of 

 America. 



The Phyllopoda deserve consideration from a geological stand- 

 point, a representative of Apus having been met with in the 

 Lower Cambrian (Protocaris Marshii) of Vermont, U.S. Some 

 of the living genera are naked (BrancJiipus and Artemia), but in 

 most the front portion of the body is protected by a shield-like 

 carapace {Apus), or it may be enclosed, as in Esiheria, in a bivalve 

 sheU. 



The fossil remains of these bivalved phyllopods of the genera 

 Estlieria and Leaia were described by Prof. T. Rupert Jones as far 

 back as 1862 in the Palseontographical Society's volume xiv. (for 

 1860), where he figures and describes 19 species ranging from the 

 Old Red and Carboniferous upwards. 



The most ancient of these shield-bearing crustaceans, originally 

 placed with the phyllopoda and having a single modern analogue 

 (Nebalia), have now, by general consent, been removed and placed 

 under the order Phyllocaeida, a name suggested by Dr. A. S. Pack- 

 ard in 1879 (' American Naturalist,' vol. xiii. p. 128). 



The fossil forms referred to this order were originally studied 

 and noticed by M'Coy, Salter, Barrande, Clarke, and myself, 

 and have subsequently been fully described in the volumes of the 

 Palseontographical Society by Prof. T. Rupert Jones and myself 

 (1888 et seq.). 



Metschnikoff, who studied the embryology of Nebalia, considered 

 it to be a ' phyllopodiform decapod.' Besides the resemblance to 

 the decapods, 'there is also a combination of copepod and phyllopod 

 characteristics. The type is an instance of a generalized form, and 

 is of high antiquity, having made its appearance in Cambrian times, 



