CLASS II ACERATA 669 
Cyclosphaeroma, Woodw., in the Great Oolite and Purbeck ; Palaega, Woodw. (Fig. 
1409), in the brackish water marls 
(Oligocene) of Paris and the Isle of 
Wight. Hosphaeroma, Woodw. (Fig. 
1410), and several Oniscids are found 
in the Miocene of Oeningen, Baden, 
and in amber. 
[The different orders of Malacostraca, 
with the exception of the Phyllocarids, 
have been revised by Professor J. S. 
Kingsley, of Tufts College, Massachusetts. 
He has also drawn up the diagnoses of 
the larger groups of Arthropods, and the 
entire MS. for this phylum has received 
the benefit of his expert criticism.— 
TRANS. | 
Class 2. ACHRATA. 
Kingsley. 
Primitively branchiate Arthropods 
in which the branchial folds function as 

gills or as lungs, or become metamor- Fic. 1409. 
phosed into air-tubes (tracheae) penetrat- . Pulaega serobiculata, v. Frc. 1410. 
= c Eee 6 Ammon. Lower Oligo- : 
umng the body. The body is divided into cene; Haring, Tyrol. an, A, Eosphaeroma Brongniarti, 
any ; Antennae; 0, Eyes; 6 M.-Edw. Middle Oligocene ; 
two MOSS cephalothorax and abdomen, Hindermost ” swimming- Butte de Chaumont, near Paris. 
the line between the two passing behind foot; I—VII, Thoracic — 3/; (after Woodward). B, Frag- 
erent . lo Cevhal segments ; 1—6, Abdomi- ment of matrix, !/; (after 
é swcth purr of appenaages. Eplavo- nal segments. Quenstedt). 
thoracic segments usually coalesced, those 
of the abdomen either free or fused. Frequently a post-anal spine is present. Antennae 
lacking ; genital openings upon the first abdominal somite; midgut long ; spermatozoa 
motile ; development without nauplius or zoea stages. 
The sole living representative of this group, Limulus, has long been regarded as occupying 
a peculiarly isolated position among Crustacea. Its Arachnidian aflinities were long ago 
pointed out by Straus-Diirckheim, and additional reasons for removing the Merostomes from 
association with the Crustacea have been brought forward by later writers, among whom may be 
mentioned H. and A. Milne-Edwards, Dohrn, Lankester, van Beneden, Kingsley, and Laurie. 
The points most relied upon for the association of Merostomata and Arachnida are: (1) 
the numerical homologies of the segments and appendages ; (2) the exact homologies existing 
in the respiratory organs ; (3) the fact that the cephalothoracic appendages are pediform, the 
basal joints serving as jaws ; (4) the presence of true nephridia opening in the base of the 
third or fifth pair of appendages, or in both ; (5) genital openings in the seventh (or more 
probably eighth) segment of the body ; (6) extreme length of the midgut ; (7) presence of an 
internal structure, the entosternite ; (8) inclusion of the ventral nerve cord and its nerves in 
the sternal artery and its branches ; (9) the close similarities in the central nervous system 
(Kingsley). 
The Acerata are divided into two groups—Merostomata and Arachnida. 
Sub-Class 1. MEROSTOMATA. (Dana.) Woodward.’ 
Six pairs of ambulatory limbs about the mouth, the first of which in some, if not in all, 
cases terminate in chelicerae. The rest serve as organs of locomotion, and their covxal 
' The best bibliographies of Merostomata, including also historical reviews of the group, are to 
be found in the following memoirs :— Woodward, H., A Monograph of the British Fossil Crustacea of 
the Order Merostomata (Palaeont. Soc. pp. 21-30), 1866-78.—Packard, A. S., On the Carboniferous 
Xiphosurous Fauna of North America (Mem. Nat. Acad. Sci. III. pp. 158-156), 1885. 
