20 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 57 



Sub-Class MEROSTOMATA 



Order Eurypterida 



Cephalo-thorax long. 



Cephalo-thorax with six (6) pairs 

 of appendages ; the anterior 

 pair chelate antennae, and the 

 posterior pair, long, strong 

 swimming legs. 



3. Epistoma present in Pterygotus 



where it is narrow. Metas- 

 toma large. 



4. Six anterior abdominal segments 



bear leaf-like branchial ap- 

 pendages. 



5. Surface of test with scale-like 



ornamentation. 



6. Terminal segment a simple lan- 



ceolate or spatulate telson. 



Sub-Order Limulava 



1. Cephalo-thorax short. 



2. Cephalo-thorax with five (5) 

 pairs of appendages ; the an- 

 terior simple antennae, the 

 third pair multi-chelate, and 

 the posterior pair short, the 

 outer joint serving as a bran- 

 chial organ. 



3. Epistoma large. Metastoma un- 

 known. 



4. Nine anterior abdominal seg- 

 ments bear leaf-like branchial 

 appendages. 



5. Surface of test smooth or with 

 imbricating lines, as in many 

 of the Trilobita. 



6. Terminal segment a caudal fin 

 formed of a central expanded 

 telson and one or more swim- 

 merets on each side. 



Differences other than those tabulated Avill probably be found 

 when more of the detailed structure of the Limulava can be de- 

 termined. 



The sub-order Limulava, as represented by the genus Sidneyia 

 with its four pairs of cephalo-thoracic appendages and simple anten- 

 nae, approaches the Trilobita, which has a similar scheme of ce- 

 phalic appendages. In both, the antennae are large and simple, 

 jointed, sensatory organs. The branchiae of Sidneyia also suggest 

 the broad, thin joints of the exopodite of the trilobite's legs with 

 their branchial fringes. For comparison, the branchial fringes of 

 Neolcmis serratus, a trilobite associated with Sidneyia inexpectans, 

 are illustrated on pi. 6, figs, i and 2. 



The branchial lamellae of Pterygotus also have branchial fringes 

 as well as the leaf-like, oval lamellae, as illustrated by Henry 

 Woodward.^ 



The short cephalo-thorax of Sidneyia is found also in Strabops 

 thacheri Beecher (p. 19) from the Upper Cambrian, a form that may 

 have had but five pairs of movable, cephalo-thoracic appendages. 



^ Monogr. British Fossil Crustacea, Order Merostomata, 1866-1878, pi. 11, 

 fig. 2b; pi. 12, figs. la, id. 



