432 Prof. E. Ray Lankester o?i the 



killen, consisting of eightj-fivc vertebras, with a spine situated 

 above the fortieth from the anterior end, and with conse- 

 quently forty-five vertebra beyond it; this number would 

 probably constitute nearly the whole of the caudal extremity, 

 and is fewer by four only than in its living representative. In 

 the specimen figured on plate vii. of the decade mentioned 

 the spine extends above the fifteenth vertebra. In the one 

 now being described the anterior dorsal spine is over the 

 eighteenth or nineteenth vertebra ; there can be no doubt, 

 however, that its proper position must have been further back, 

 because the fin to which it was attached is far behind the 

 spine. From analogy it would be supposed that the spine 

 occupied a position halfway between its present situation and 

 that of the fin ; and as this would place the spine above the 

 twenty-second or twenty-third vertebra from the head, which 

 is, as already indicated, the point inferred from the comparison 

 with the recent fish, there remains little doubt that such was 

 its actual position. 



Locality. All the specimens hitherto described, including 

 the one which is the subject of this paper, are from the Lias 

 at Lyme Regis. 



EXPLANATION OF PLATE XX. 



Fig. 1. Palceospinar jn-iscus, Egerton (nat. size). 



Fiff. 2. Dermal tubercles or sliagreeu on ventral fin (x 2o). 



Fig. 3. Ditto on ventral portion of body behind the ventral fin (x 25). 



Fig. 4. Ditto on pectoral fin near the base of the anterior margin ( x 25). 



XLIV. — On the originally Bilateral Character of the Benal 

 Organ of Prosohranchia, and on the Homologies of the Yelk- 

 sac of Cephalopoda, ^j E. Ray Lankester, M.A., F.R.S., 

 Jodrell Professor of Zoology in University College, 

 London. 



Two recent memoirs on molluscan morphology touch upon 

 matters which have formed the subject of investigations by 

 me, and which I have formerly discussed in the pages of this 

 journal. I am therefore anxious to make a few remarks 

 on the matters in question in the same place as that in which 

 I first wrote of them. 



I. Dr. J. W. Spengel, in a very interesting essay (Zeitschr. 

 wiss. Zool. vol. XXXV.) entitled " Die Geruchsorgane und das 

 Nervensystem der Mollusken," refers to a note by me " On 

 some undescribed Points in the Anatomy of the Limpet {Fa- 



