6 BRITISH PALAEOZOIC PHYLLOCARIDA. 



The order Phyllocarida has been thus denned : 



Phyllocarida, Packard (1879). Body long, with five cephalic, eight thoracic, 

 and eight abdominal segments, with a thin or chitinous skin ; generally covered with 

 a bivalved shell having a movable 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 is composed of eight 

 large segments 1 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 

 very small. The abdomen terminates in setaceous filaments, or in a telson divided 

 into three or more parts. (Zittel, ' Handbuch der Palaeontologie,' Munich, 1885.) 



In 1880 Professor Claus, ' Lehrbuch der Zoologie,' writes, " This remarkable 

 form (Nebalia) was for a long time regarded as a Phyllopod, and in many of its 

 characters it represents a connecting link between the Phyllopoda and the 

 Malacostraca. The structure and segmentation of the head and thorax resemble 

 that of the Malacostraca, but the terminal region of the abdomen does not present 

 the special form of a caudal plate or telson. In Nebalia we probably have to do 

 with an offshoot of the Phyllopod-like ancestors of the Malacostraca, which has 

 persisted to the present time." He adds, " Nebalia is best placed in a special group 

 Leptostraca, between the Entomostraca and Malacostraca. The Palaeozoic genera 

 Hymenocaris, Peltocaris, &c. would have to be placed in such a group." 8 



" It is," writes Professor Claus, " in the highest degree probable that all these 

 [Palaeozoic Phyllocarida] are not true Phyllopods, but have belonged to a type of 

 Crustacea, of which there are no living representatives, but which, taking their 

 origin from forms allied to the lower types of Entomostraca, have prepared the 

 way for the Malacostracan type. Such a connecting link, which has survived to 

 the present day, we evidently find in the genus Nebalia." 5 



In his ' Handbuch der Palaeontologie,' Munich, 1885, Professor Dr. K. A. Zittel 

 adopts Packard's order Phyllocarida, but places it under the Malacostraca, and 

 between the Edriophthalmia and the Merostomata. 



In his article on the Palaeozoic allies of Nebalia, Dr. A. S. Packard, jun., thus 

 sums up the Phyllocarida : " From our total lack of any knowledge of the nature of 

 the limbs of the fossil Phyllocarida, we have to be guided solely by analogy, often an 



1 The abdomen is nine-jointed, unless the last somite be considered as the telson (it is post-anal). 

 It is a long and slender segment, and bears two very long narrow setigerous cercopods, closely resem- 

 bling those of the Copepoda. 



2 Claus, translated by Sedgwick (Cambridge), p. 448 (footnote) ; 8vo, 1884. The Leptostraca 

 (Claus) are thus defined : " Crustacea with thin folded carapaces, mostly bivalved, under which all the 

 thoracic rings remain as free segments " (Zittel, ' Handb. Palaeontol.,' 1885, p. 655";. 



3 Claus, in ' Siebold und Kolliker's Zeitschrift,' vol. xxii, 1872, p. 329. 



