6 NOTES AND COMMENTS [januahy 



selection, environment, and growth," as if all acted after a precisely 

 similar fashion. Horns, we are told, may arise as the direct result of 

 the action of the environment, but if they are confined to one sex it is 

 "evidently a case of sexual selection." We do not suppose that Mr. 

 Beecher regards this " force " as a kind of " Eed Queen," whose fiat 

 " Off with his head," or her horns, can be relied upon to assist the other 

 " forces and laws " when they get into difficulties, but it is surely 

 unfortunate that his language should lend itself so readily to such 

 misapprehension. 



Eyes in Bivalve Mollusca. 



The Acephala, or headless Mollusca, as the Pelecypoda have been- 

 termed, whilst possessing eyes in the normal position near the mouth 

 when in the larval condition, have hitherto been supposed to be quite 

 devoid of such organs when adult. Some of them, it is true, have 

 developed organs of vision elsewhere, for instance, in the mantle-margins 

 (Pecten, etc.), or at the extremities of the siphons (Cardium), but these 

 have no connection with true cephalic eyes. 



Dr. Paul Pelseneer has, however, recently discovered the existence 

 of true cephalic eyes in certain groups (C. B. Ac. Sci. cxxvii. (1898), 

 pp. 735 and 736). Since his account is short we give a full trans- 

 lation : — 



There exists in adult Lamellibranchs a pair of distinct and well-formed 

 cephalic eyes. They consist of pits with pigmented walls filled by a cuticular 

 lens, and they are thus intermediate in their structure between those of Trochus 

 and Patella. 



These organs appear to be peculiar to most of the Mytilidae (Mytilus, Liiho- 

 domns, Modiolaria) and to the allied genus of Avicula proper (exclusive of 

 Meleagrina). 



They occur both in the larva and the adult, but only do not make their 

 appearance in the former (Mytilus) after the formation of the first branchial 

 filaments. 



They are situated at the base and on the axial face of the first filament of 

 the internal branchial lamella, and are innervated from the cerebral centre. In 

 the larva they are situated outside the posterior margin of the velum. 



They appear to be homologous to the larval eyes of Chitons, which are also 

 outside the velum and at the sides of the cephalic region, but they are not 

 homologous to the cephalic eyes of Gasteropods, which arise within the velar 

 area, 



Those Cretaceous Gryphaeas. 



The extreme variability of the Gryphaeas has always rendered them 

 difficult of study, and in no place has this difficulty been more fully 

 understood than in the Texan Eegion. It is therefore with considerable 

 satisfaction that we call attention to the lolst Bulletin of the United 

 States Geologiccd Survey, in which Messrs. E. T. Hill and T. Wayland 

 Vaughan have attacked the Lower Cretaceous Gryphaeas of the 

 Texan Eegion and evolved some order out of chaos. These mollusca 



