M r. J. E. Gray on Prof. Loven's Homologies of Mollusca. 215 



XIX. — Observations on Professor Loven's Homologies of Mol- 

 lusca. By J. E. Gray, F.R.S., V.P.Z.S. &c. 



Professor S. Loven, in a very instructive paper on the Deve- 

 lopment of Bivalve Mollusca, in the ' Swedish Transactions ' 

 for 1848, pubUshed in 1850, has given a table showing his idea 

 of the changes which the — 1. velum, 2. eyes, 3. foot, 4. byssus, 

 5. pallial siphon, 6. shell, and 7. the internal plates of shells 

 undergo in the different orders of Mollusca, and the organs 

 which he regards as homologous to one another in the different 

 orders. This paper contains many most interesting observations, 

 showing very great research, and it must be considered as one 

 of the most valuable contributions to the study of these animals 

 which has been recently published ; at the same time there are 

 certain conclusions in it which are quite contrary to my own re- 

 searches. As Professor Loven has specially requested my opi- 

 nion on them, I venture to state the reasons which induce me to 

 dissent from them, hoping to elicit truth by the discussion. 



1 . Professor Loven regards the operculum of the Gasteropodous 

 Mollusca as homologous with the byssus of the bivalve ; he de- 

 scribes it thus : " Byssus lamellis ssepe spiralibus coalitis oper- 

 culum officiens.'^ 



In the '^ Annals and Magazine of Natural History,' June 1850, 1 

 have given the reasons which induced me to believe the opercu- 

 lum to be the homologue of the second valve of the bivalve shell, 

 and the opercular mantle the homologue of the second flap of 

 the mantle of those animals. Further observation and compa- 

 rison have only served to confirm my views. 



In the latter parf of the paper, which alone I profess to be able to 

 read, there are no reasons given to show why Dr. Loven regards 

 the operculum and the byssus as homologous. I can only suggest 

 two : first, that the operculum is often horny, and of the same 

 horny consistence as the byssus ; secondly, they are both placed 

 on the foot. To these I must reply, that, though the substance 

 of the operculum is also similar to the periostraca of the shell, 

 and like the shell this horny part of the operculum is often 

 covered with a shelly coat ; and though the operculum and the 

 byssus are both placed on the foot, they are situated in very dif- 

 ferent parts of that organ. The byssus is always placed on the 

 hinder part of the lower edge ; while the operculum is uniformly 

 placed on the back part of its upper surface. There is also no 

 similarity either in the oeconomic use or in the development of 

 these parts. The byssus is merely used to attach the animal 

 to a particular locality ; while the operculum is uniformly and 

 only used to cover up and protect the animal w^hen it is with- 

 drawn into the shelly case. The byssus, which is only required 



