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The Aglaspida are Merostomata having a phosphatic exoskeleton consisting of a longitudinally trilobed dorsal shield and a longitudinally cleft postventral plate. The dorsal shield comprises a cephalothoracic shield, or carapace, and 12 abdominal segments, all freely articulating in the Aglaspidae S. A. Miller, with those of the postabdomen coalesced in the Beckwithiidae nov. In the Aglaspidae, the twelfth segment bears a long, spiniform posterior extension, the telson spine.

The cephalothorax bears a pair of compound eyes and six pairs of uniramous, stenopodous, jointed appendages; the first pair (as determined from Aglaspis) is rather strongly chelate, the remaining five simple and unspecialized; the eyes vary decidedly in size and position in different genera. An occipital segment is distinguishable in some genera. Indications are that the ventral surface of the cephalothorax was covered by a tenuous ventral membrane similar in composition to the dorsal shield.

The abdomen in the Aglaspidae is divided into a praeabdomen and a postabdomen, the latter two- or three-segmented. Beneath the post-abdomen (excepting the telson spine) lies the ventrally facing postventral plate. At least the first seven abdominal metameres (as determined from Aglaspis) bear jointed appendages similar to the posterior five of the cephalothorax. In all segments of the abdomen, articulation took place along the entire anterior edge.

The respiratory system is unknown.

On the basis of the present study, the writer feels justified in removing the aglaspids from the Synxiphosura, a very heterogeneous group in contrast to the closely knit group of aglaspid genera. Walcott’s order Aglaspina, . . .

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