496 W. D. Lang — Cretaceous Cheilostome Polyzoa. 



In the head-shield of Nevadia the glabella diminishes anteriorly, 

 and is divided by complete transverse furrows into five segments, of 

 which the first is longer than the others, and sends off lateral stout 

 eye-lobes which probably bear a visual area 1 externally and distally. 

 In Conocoryphe the glabella is almost equally primitive, but its 

 anterior segment is small and its furrows oblique and not continuous 

 across. The absence of eyes is probably secondary.* In Ellipso- 

 cephalus the glabella has already attained a high degree of speciali- 

 zation, for it is quadrate in outline and may have all its furrows 

 smoothed away. It possesses eyelines and elongated eyes, both of 

 which are primitive characters. 



In Nevadia there is no facial suture, but its potential position 

 (Figs. 2a and 3a) is marked by the eye. From this it will be seen 

 that the free cheek region of Nevadia is of great breadth, whilst the 

 fixed cheek is particularly narrow. In Conocoryphe (Fig. 2b) and 

 Ellipsocephalus (Fig. 2c) these relative proportions are reversed, for 

 the free cheek is narrow and marginal whilst the fixed cheek is broad. 



The fixed cheek largely represents the pleural portions of the hind 

 segments of the glabella. Its insignificance in the one genus and 

 its great size in the other genera accord with the morphological 

 characteristics of the trunk segments. 



Bernard 3 considers that in the evolution of Arthropods from an 

 annelid ancestor pleurae appeared first on the head segments, and 

 that the early Arachnids and Trilobites specialized by "the con- 

 tinuation of the original head pleurse along the whole length of the 

 body". In Nevadia (Fig. 2a) this specialization has not advanced so 

 far as in Conocoryphe and JEUipsocephalus. Leaving the pleural spine 

 out of account the pleural lobes of the first few trunk segments of 

 Nevadia are not so imposing as in the other two genera. They then 

 diminish in size posteriorly and finally disappear, so that the hinder 

 segments consist of axis only. In other genera (Figs. 2b, c) the 

 pleural lobes are well developed even in the pygidium. 

 {To be concluded in our next Number.) 



III. — On some new Uniserial Cretaceous Cheilostome Polyzoa. 

 By W. D. Lang, M.A., F.G.S. 



(Published by permission of the Trustees of the British Museum.) 



(PLATE XVII.) 



I. Rhammatopora, 4 new genus. 



IN 1890 Yine described Membranipora gaidtitia, 5 a uniserial 

 Cheilostome from the Cambridge Gault. A peculiarity of this 

 species was ' a puckering' or 'folding-in of the wall below the area' 



1 Vide Walcott on Olenellus gilberti, in Smiths. Misc. Coll., vol. liii, p. 327, 

 1910. The discovery of the presence of a visual area in Olenellus gilberti 

 is against Wood's suggestion (op. chV, p. 232) that the eye-like lobe of Olenellus 

 " is really of the nature of a pleura ". 



2 Nevadia, Burling ia, Marella, Nathorstia, are all more primitive than 

 Conocoryphe, and yet they all possess well-formed eyes. Again, secondary 

 loss of eyes is now definitely established in some families, e.g. Trinucleidse. 



3 Q.J.G.S., 1895, p. 359. 



4 to panixa, ' a seam.' 



5 Vine, 1890, Quart. Journ. Geol. Soc, vol. xlvi, pp. 484, 461. 



