A DOUBTFUL DATE 423 



Family 2.—Tylidce. 



The segments of the perseon, except the first, have the 

 side-plates marked off by a distinct suture. The pleon 

 consists of six segments, of which the first two are very- 

 short and narrow. The first antenna are small, adpressed, 

 one- to two-jointed. The components of the mandibular 

 spine-row are numerous. The first maxillae have three 

 setae on the inner plate. The second have only one plate. 

 The ' palp ' of the maxillipeds is two-jointed. The first 

 pleopods are wanting (or retain a doubtful rudiment in 

 the male). The second, third, fourth, and fifth pairs are, 

 according to Budde-Lund, all branchial, single-branched. 

 Von Ebner, however, says : — ' The peduncular plates of the 

 branchiferous caudal appendages in l^ylos are not essen- 

 tially different from those of other Oniscoida ; they carry 

 as well the branchial-operculum as the branchial-sack.' 

 Of the respiratory apparatus in this famUy, he remarks : — 

 ' The four developed branchial-opercula all contain rami- 

 fied air-cavities. These consist essentially of a sack 

 enclosed between the two leaves of the branchial-oper- 

 culum, which divides into a number of microscopic csecal 

 tubes. Quite similarly constructed air-cavities occur also 

 on the two first, often on all, of the branchial-opercula of 

 other Oniscoida. These, however, are distinguished from 

 those of the Tylinse, by the circumstance that it is not 

 openings of the under surface of the operculum, but cross 

 slits on its hinder margin, that lead to them.' The uropods 

 of the Tylidffi are something like those of the Valvifera, 

 but here the folding doors form an operculum not over 

 the pleopods but over the anal opening of the telson ; they 

 carry a minute single branch, and are completely covered 

 dorsally by the terminal segment of the pleon. 



Tylos, Audouin, 1825? This being the only genus 

 has the characters of the family. The first verbal descrip- 

 tion was printed by Audouin in his ' Explication sommaire 

 des planches des Orustaces de I'Egypte et de la Syrie.' 

 He states that Tylos was a manuscript name given to the 

 31 



