22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 57 



Abdomen with twelve segments, the anterior nine of which have 

 a pair of ventral appendages to which the branchiae are attached ; 

 the posterior segment has a central spatulate-shaped section that, 

 combined with swimmerets, forms a strong caudal fin. 



The description of the branchiae will be found under the descrip- 

 tion of Sydneyia, the typical genus of the Limulava. 



Observations. — The sub-order I.imulava differs from the Euryp- 

 terida, to which it is most nearly related, in having a large epistoma 

 similar to that of the Trilobita ; in not having a metastoma, chelate 

 antennae, and swimming cephalic appendages ; and in having a broad 

 fan-shaped caudal fin, and branchial appendages more or less unlike 

 the lamellar branchiae of the Eurypterida and Xiphosura. 



Family SIDNEYIDAE, new family 



Cephalo-thorax small, without lobes, eyes marginal ; ventral side 

 with large epistoma, five pairs of movable appendages, the gnatho- 

 bases of the three posterior pairs forming organs of manducation. 

 Abdomen twelve- jointed, the three posterior segments annular and 

 narrow, the terminal one forming, with lateral swimmerets, a fan- 

 like tail ; nine anterior segments with a pair of ventral branchial 

 appendages on each ; the three posterior segments without ventral 

 appendages. Surface smooth or ornamented by narrow, irregular, 

 fine, imbricating ridges. 



Genus SIDNEYIA, new genus 



Body elongate, broadly oval in outline, attaining as now known 

 a length of 17 cm., covered with a thin dorsal shield or crust, divided 

 into a short cephalon, broad anterior abdomen, and narrow pos- 

 terior abdominal portion. Cephalo-thorax transverse, short, de- 

 pressed convex, as compressed in the shale, with broadly rounded 

 frontal margin and antero-lateral angles ; margins smooth. Eyes 

 reniform and situated near the postero-lateral outer margin. Ventral 

 side with five pairs of m,ovable appendages. The anterior pair are 

 large, long, simple, jointed antennae ; second pair, slender and 

 jointed ; third pair with numerous spines on the front side of the 

 joints and with variously developed chelate-like outer joints (see 

 pi. 4) ; fourth pair, slender and jointed ; fifth pair with a large 

 basal joint, and an outer, broad joint or palp that is fringed with 

 fine branchial setae or spines. A large epistoma is attached to the 

 front margin and back of it the gnathobases of the appendages, the 

 three ^ posterior pairs of which form the organs of manducation. 



^ See description of species, 5". incxpcctans (p. 25). 



