Oq the Visual Organs in Lamellibranchiata. 451 



the eye of tliis form is essentially the same as that oi Pecten and differs 

 fiom it only in a few particulars. 



The paper of Delle Chiaje ^ referred to by Hensen 2 and two 

 short papers of J. Chatin 3 referred to by Hickson I have not been 

 able to find, and have to depend on the references for my knowledge 

 ofthem. 



The apparent disorder in the following pages may be perhaps ex- 

 plained here. I first examined in detail the histology of Solen vagma 

 and S. ensis. I was then led to take up the other forms, namely: 

 Ostrea, Mya, Venus. Mactra etc. , as I could obtain them, and when 

 fully satisfied with my results I went through the alcoholic collection 

 of Lamellibranchiata of the Academy of Natural sciences, whieh I am 

 sorry to say is not rieh, neither are the specimens well preserved, and 

 I am consequently in doubt as to many forms. 



I then arranged the speeies that I had examined by families in 

 their Order beginning with the lowest: thus reversiug the order of 

 Tryon 4 in his Conchology. 



This accounts for the fact that the detailed descriptions are scat- 

 tered about. 



I may here express my thanks to Mr. Tryon for his kind assistance 

 in determiuing for me many of the speeies. 



In entering now upon the special part of this paper I intend taking 

 up in order the difi'erent forms of Lamellibranchiata that have been 

 examined by myself and others. Owiug to the bad conditiou of a num- 

 ber of the forms that I have examined I can give but fragmentary notes 

 concerning them. 



Taking up first the order Asiphonida I will first consider the 



Farn. Ostreidae, 



Ostrea virginica Linn. When the anterior border of the mautle of 

 this speeies is examined it is found to be fringed with innumerable little 

 tentacles ranging from 1 mm to 2 mm in length and arranged in three 



1 Delle Chiaje, Osservazioni anatomiche suU' occhio umano. 



2 V. Hensen, Über das Auge etc. 1865. p. 158 and 222. 



3 J. Chatin, Bulletin de la Société Philomatique. Paris 1877. 



* G.W. Tryon, jr., Structural and systematic Conchology: an introduction 

 to the study of the Mollusca. Philadelphia 1884. Vol. III. p. 116 et seq. 



30* 



