4 BRITISH PALAEOZOIC PHYLLOCARIDA. 



The Phyllocarida. — We have long held the opinion that the expanded disc-like 

 shields, such as Peltocaris, Discinocaris, Aftycliopsis, and some others, were probably- 

 related ancestrally to the larval or adult forms of Phyllopods like Apus, Lepidurus, 

 &c., whilst the relationship between the living Nebalia and the numerous genera 

 of Palaeozoic Pod-shrimps does not necessarily preclude us from considering these 

 forms as still belonging to the Entomosteaoa, although placed in Packard's order 

 Phyllocarida. 



As to ornamentation, the concentric striae, marking lines of growth, appear to 

 correspond most closely in character and origin with the similar decoration 

 observable on the valves of Esther ia, Limnadia, &c., so that their absence upon the 

 carapaces of Apus and Nebalia does not necessarily prove that shields so ornamented 

 cannot be deemed to belong to Crustacea or even to the Phtllopoda ; whilst many 

 of the carapaces of the fossil genera, e. g. Dithyrocaris, Geratiocaris, &c., have 

 either concentric or anastomosing strise covering the entire surface of their cara- 

 paces ; and these forms are related to Nebalia, which has a smooth carapace 

 destitute of ornamentation. 



Claus and Gerstaeker are of opinion that Nebalia is not a Phyllopod. Because 

 Nebalia during its embryonal life (whilst still in the egg) passes through the 

 " Nauplius " and " Zoca stages," which in Decapods occur partly in the free state, 

 it has been regarded by some as a " Phyllopodiform Decapod." The potentiality of 

 a form to attain to a higher existence seems to be here mistaken for actuality. 

 Since it never attains a higher development, as an adult, than that of a Phyllopod, 

 and has no retrograde metamorphosis, may we not with as equal reason regard 

 Nebalia as a highly-organised Phyllopod, as to assert that it is a Decapod arrested 

 at the Phyllopod stage ? 



All who have studied the Phyllopoda have been struck by the peculiar points 

 of special interest to be observed in Nebalia} 



Milne Edwards, in his ' Histoire Naturelle des Crustaces ' (1840), places Nebalia 

 in the family Apusidce among the Phyllopods ; at the same time he remarks, " The 

 Nebaliee are very singular little crustaceans, which, by reason of their stalked eyes^ 

 and their carapace, approach the Podopiithalmia ; they do not, however, possess 

 branchiae, properly so called, but they respire by the aid of their thoracic feet, 

 which are developed into membranaceous and foUaceous appendages. They 

 resemble in many respects, and establish a passage between Mysis and Apiis." 



1 For a very full account of Nebalia, see the ' Twelfth Annual Report of the United States 

 Geological Survey,' Part I, " Geology, Palaeontology, and Zoology," 8vo, 1883 (Washington), " A 

 Monograph of the Phyllopod Crustacea of North America, with remarks on the Order Fhyllocarida," 

 by A. S. Packard, jun., pp. 295 — 592, and plates i — xxxix. Also the ' American Naturalist ' for Oct., 

 Nov., and Dec, 1882, vol. xvi, pp. 785, 8G1, 945 ; and G. O. Sars, ' Challenger Reports,' 1885 and 1887. 



* Pedunculated eyes are also present in Branchipus and Artemia, so that the stalked eyes of 

 Nebalia can scarcely be regarded as an essentially distinctive character. 



