326 Dr.F. B. Cowper Reed — The genus Homalonotus. 



sufficiently distinguish this subgenus from the typical jET. Serscheli 

 group. The large pre-sutural area and upturned prora, only known 

 from the specimen of H. hifurcatus, sp. nov., in the Sedgwick Museum, 

 are likewise peculiar. But they recall the structure of the head- 

 shield of JDipUura Dekmji. With regard to the shape and characters 

 of the pygidium and general well-marked trilobation, we see affinities 

 with the Ordovician Brongniartella. The paired tubercles on the 

 glabella and other features of the head-shield of H. armatiis suggest 

 that this species when completely known may have to be placed in 

 this section. The Rhenish J3^. acuhatus has a pj^gidium apparently 

 much like that of the new British species H. htfurcatus. The only 

 well-known forms are from Devonshire. 



9. Parahomalonotus, nom. prop. 



Head-shield semicircular, transverse ; facial sutures uniting in 

 regular wide-arched commissure close to anterior margin. Thorax 

 with axis obsolete and trilobation quite lost. Pygidium rounded, 

 semicircular, or semi-oval, with entire margin ; trilobation. more or 

 less indistinct, but segmentation well marked ; border not crossed by 

 pleurae. Surface ornamented with coarse tubercles and granules (or 

 smooth). 



Type. — H. Gervillei, De Verneuil.^ 



Eange. — Lower Devonian. 



Distribution . — Europe. 



EX.\MPLES. 

 H. pradoanus, De Vern. ?jff. Icevicauda, Quenst. 



H. Barratti, Woodw. ? H. ohtusus, Sandb.^ 



H. Hausmanni, Eouault. ?-H". multicostatus, Koch.* 



?S'. planus, Sandb. 

 Eemakks. — This subgenus is characterized by the regular curved 

 union of the facial sutures close to the anterior margin (reminding 

 us of the conditions in JEoJiomalonotus and Brongniartella), by the 

 disappearance of the trilobation in the thorax (as in the Biimastus 

 group of Illcenus), and by the regular rounded outline, obsolescent 

 trilobation but distinct segmentation of the pygidium. It looks as if 

 these characters must be due to reversion, as no Silurian forms are 

 known to connect the Ordovician species with this Devonian group. 

 The four last-mentioned examples in the above list differ from the 

 typical members of this subgenus in several minor respects, especially 

 in being smooth, but for the present may be best referred to this 

 subgenus. 



10. Dipleura, Green. 



Head-shield subtriangular, with large pre-sutural prora. Facial 



sutures uniting in front of glabella by straight transverse commissure, 



but continued directly forwards without deviation into the epistomal 



sutures bounding the prora. Thorax with faint trilobation. 



1 Tchichatcheff, Asie Mineure, Paleont., p. 448, pi. xx, fig. 1, 1866 ; Bayle, 

 Explic. Carte Gtel. France, iv, atlas, pi. ii, figs. 1, 3, 6, 1878. 



^ Sandberger, Verstein. rhein. Schicht. Nassau, t. ii, p. 26, figs. 6-6d, 1856 

 Koch, op. cit. , p. 49, pi. vi, figs. 1-4. , 



^ Koch, op. cit., p. 52, pi. vi, figs. 5-9. 



