INCISUEA (sCISSUEELLA) LVTTELTONENSIS. 19 



appearances lend support to tlie reticular theory of the con- 

 stitution of striped muscle-fibre, and are inconsistent with 

 the opposing theoi-y of sarcomeres. 



The mandibles occupy the usual position at the sides of the 

 mouth, and are composed of a number of plates or "tesserae" 

 as d escribed by Vay ssiere for S c i s s u r e 1 1 a c o s t a t a. Randies 

 has shown that each tessera is the product of a single epithe- 

 lial cell in Trochus, and the .same is evidently the case in 

 Incisura. The radular sac occupies the usual position. 

 Lying at first between the upper horns of the odontophoral 

 cartilages it maintains a mediau position to the posterior end 

 of the buccal bulb, and then curves to the right between the 

 I'ight oesophageal pouch and the pedal ganglia and soon ter- 

 minates in a swollen bilobed extremity lying on the right side 

 of the liEemocoele. The radular teeth are represented in fig. 

 20. Tlie centrals are squarish, with an expanded basal plate; 

 their anterior margins decurved, and furnished with five very 

 distinct and sharp-pointed denticulatious. The next three 

 teeth (medio-laterals) are oblong, with decurved denticulate 

 margins ; they decrease somewhat in size from within out- 

 wards. Tlie next tooth is much smaller, has a somewhat 

 sigmoid curvature, a thickened base, a narrow neck, and a 

 single recurved marginal denticulation. The next tooth is 

 very large, shaped somewhat like a rake with a crooked 

 handle, its expanded mai-gin decurved and bearing about a 

 dozen denticulations. Then follow the marginals or uncini, 

 which are numerous, curved, slender, with expanded and re- 

 curved denticulate margins. The radular formula may be 

 written : 



00 1 (4 + 1 + 4) 1. 00 



Vayssiere has given a good figure of the radula of Scis- 

 surella costata, which is similar to but differs in small 

 details from that of Incisura. The radula of the ScissurellidtB 

 is usually described as resembling that of Trochus, but it is 

 much more nearly like that of the Fissurellidse. A reference 

 to Thiele's figures in the concluding chapter of Troschel's 

 ' Gebiss der Schnecken' shows that the radula of Incisura 



