and on Nebalia, and its Allies. 349 



In 1879, Dr. A. S. Packard, jun., in the " American Naturalist," 

 vol. xiii. p. 128, proposed that Nehalia and its fossil allies should 

 be placed in a new order, which he proposed to name the Phyllo- 

 OARiDA. Dr. Packard writes : — 



" The Nehaliadce, represented by the existing genus Nehalia, have 

 generally been considered to form a family of Phyllopod Crustacea. 

 Metschnikoff, who studied the embryology of Nehalia, 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 one, and 

 is of high antiquity, having been ushered in during the earliest 

 Silurian Period, when there were (when we regard the relative size 

 of most Crustacea, and especially of living Nebalicd) gigantic forms. 

 Such was Dithyrocaris, which must have been over a foot long, the 

 carapace being seven inches long.^ The modern Nehalia is small, 

 about half an inch in length, with the body compressed, the carapace 

 bivalved as in Limnadia, one of the genuine Phyllopods. There is 

 a large rostrum overhanging the head ; stalked eyes ; and, besides 

 two pairs of antennse and mouth-parts, eight pairs of leaf-like, short, 

 respiratory feet, which are succeeded by swimming feet. There is 

 no metamorphosis, development being direct. 



Of the fossil forms, Hijmenocaris was regarded by Salter as 'the 

 more generalized type.' The genera Peltocaris and Discinocaris 

 characterize the Lower Silurian Period, Ceratiocaris the Upper, 

 Dictyocaris the Upper Silurian and the lowest Devonian strata, 

 Dithyrocaris and Argas the Carboniferous Period. Our existing 

 north-eastern species is Nehalia hipes (Fabricius), which occurs 

 from Maine to Greenland. 



The Nebaliads were the forerunners of the Decapoda, and form, 

 we believe, the type of a distinct order of Crustacea, for which the 

 name Phyllocauida is proposed." 



The order Phyllocarida is thus defined : — 



Phyllocarida, Packard (1879). Body long, with 5 cephalic, 

 8 thoracic, and 8 abdominal segments, with a thin or chitinous skin ; 

 generally covered with a bivalved shell having a moveable rostrum. 

 Eyes pedunculated and faceted. Upon the under side of the head 

 are two pairs of antennae ; the mandibles, and two pairs of maxillae 

 furnished ■ with palpi. The body-segments are compressed, they 

 support eight pairs of large Phyllopodiform thoracic feet. The 

 abdomen composed of eight large segments,^ provided with six pairs 

 of simple swimming-feet fringed with setae, of which the four anterior 

 pairs are the largest, and the two posterior pairs are very small. 

 The abdomen terminates in setaceous filaments, or in a telson 

 divided into three or more parts. (Zittel, " Handbuch der PalaBon- 

 tologie," Munich, i. Band, ii. Abth., iv. Lief. p. 656, July, 1885.) 



^ The Devonian Dithyrocaris Neptuni of Hall must have been some two feet or 

 perhaps more in length. 



^ The abdomen is nine-jointed, unless the last somite be considered as the telson 

 (it is post-anal). It is a loug and slender segment, and bears two very long narrow 

 setigerous cercopods, closely resembling those of the CoPEroDA. 



