NO. 26.] ARTHROSTRACA OF CONNECTICUT. 17 



The segments of the thorax increase gradually in length from 

 the anterior end to the posterior, except in Harpinia, where the 

 last segment of the thorax is shorter than the preceding on 

 account of the reduction of the last pair of thoracic legs. 

 Although typically the dorsal region of the thoracic segments is 

 smooth and evenly rounded, in a few cases it may be produced 

 to form a keel as in Epimeria and Par amp hit hoe. 



The first three segments of the abdomen are always in the 

 Gammaridea deeper than those of the thorax and have the 

 lateral margins prolonged downwards to form pleura; the last 

 three segments are small and do not have pleura. The dorsal 

 region of the abdominal segments, like the thoracic, is usually 

 evenly rounded, but more frequently than in the thoracic seg- 

 ments, it may exhibit a keel or crest as in Apherusa gracilis. In 

 Ampelisca spinipes and Byblis serrata the fourth segment bears 

 a prominent keel which ends abruptly posteriorly. The first four 

 segments of Calliopius t&viusculus are quite characteristically 

 raised posteriorly to form rounded keels, while in Dexamine thea, 

 these segments are prolonged posteriorly to form sharp, spiniform 

 projections, and in Carinogammarus the first three segments only 

 are similarly produced. Many of the Lysianassidas and Ampelis- 

 cidae exhibit a depression on the dorsal side of the fourth abdom- 

 inal segment which is more pronounced in the male than in the 

 female. In Chelura terebrans, the third abdominal .segment 

 bears a long curved process which extends posteriorly. The 

 posterior margins of the three posterior segments may be armed 

 with fascicles of spines, as in Carinogammarus and Gammarus, 

 or produced to form small spiniform projections, as in Mara and 

 Melita dentata, or the spines may be confined to the fourth and 

 fifth segments, as in Leptocheirus pinguis. 



The postero-lateral angles of the first three abdominal seg- 

 ments are usually evenly rounded but the third is especially 

 modified frequently. Thus in Ampelisca macrocephala, Unciola 

 irrorata, Sympleustes glaber, and Harpinia plumosa, it bears an 

 acute, slightly upturned projection, above which is a rounded 

 sinus, which in some cases may be followed by a rounded lobe. 

 In a few species the postero-lateral margins of the third abdom- 

 inal segment may be serrated, as in Batea secunda; or the second 

 and third may 'both be so adorned, as in Apherusa gracilis. 

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