406 THE OPISTHOBRANCHIATE 



adjacent lateral teeth of a median transverse row is represented for 

 five Plymouth Ajplysix of different sizes. Fig. 3 represents these 

 teeth in a very small specimen^ whose radula measures 1'05 mm. in 

 length by 0*G mm. in breadth, and consists of twenty transverse rows, 

 its formula^ being (8- 1-8) x 20. The central tooth consists of a 

 broad but short basilar portion, deeply excavated behind, and a 

 transverse projecting ridge, arising from the basilar portion in its 

 anterior half, directed posteriorly, and made up of five well-deve- 

 loped cusps, of which the median is the largest and has five serra- 

 tions on each side, while the internal and external lateral cusps are 

 smooth, the external cusp being smallest. The posterior excavation 

 of the basilar portion of the tooth will be referred to as the " poste- 

 rior bay ; " a similar but smaller excavation in front will be called 

 the '' anterior bay." The structure of the tooth in a very young 

 specimen being understood, the modifications entailed by further 

 growth can be most accurately shown by a series of measurements, 

 although the striking nature of the changes is more graphically 

 shown on PI. XXVIII. 



Mr. Hunt has very kindly lent me a number of the preparations 

 made by him in 1877 and 1878, so that I have been able to incor- 

 porate the results of a re-examination of them with those obtained 

 from Plymouth specimens. These results are contained in the accom- 

 panying tables. For the " species " A. punctata (Nos. 1 — 10) the 

 measurements and observations show — 



(1) That the radula may attain a size of 8"8 mm. in length by 



5'4 mm. in breadth, considerably larger than that figured by 

 Blochmann (6x4), and may consist of forty-four transverse 

 rows of teeth, the lateral teeth numbering (according to age) 

 from eight to nineteen. Blochmann gives twenty rows and 

 thirteen lateral teeth for this species at Naples, while Vays- 

 siere has observed from thirty-five to thirty-six rows and 

 sixteen lateral teeth at Marseilles. 



(2) That the basilar portion of the central teeth may increase in 



breadth (according to age) from 0'15 mm. to about 0'4mm,, 

 when it begins to be reduced in width (Nos. 9 and 10), and 

 that it increases in height regularly (with age) from 0'025 mm. 

 to 0'15 and even 02, In several of the radulae (Nos. 5, 6, 

 8, 9, and 10) this increase can be actually observed by com- 

 paring the heights of the central teeth in front and of those 



^ In my formulae for the teeth of Aplysia the three, or sometimes four, rudimentary 

 lateral teeth at the extremities of the transverse rows are always included. In comparing 

 these formulfe with those given by Mr. Hunt it should be remembered that Mr. Hunt counts 

 only the perfectly developed lateral teeth. 



