MARSHALL: ADDITIONS TO 'BRITISH CONCHOLOGY.' 251 



Guernsey, at low water, on some off-lying rocks. It has 

 its white representative. The shell resembles in shape 

 Natica tnontaaiti. 

 Rlssoa striatula var. varicosa Marsh. Having one, two, 

 and rarely three prominent ribs on the body-whorl. Found 

 occasionally with the type. 

 R. parva var. semicostata Mont. Only half ribbed. Not 

 noticed by Jeffreys, but quite worthy of a varietal name. 

 Var. semicostata is somewhat of a misnomer, as all typical 

 specimens have three smooth upper whorls and three ribbed 

 lower ones ; but in this variety the bottom whorl is smooth. 

 R. striata var. distorta Marsh. Shell more or less distorted 

 out of its normal axis. It is found everywhere with the 

 type. Rissoa striata has the faculty of putting on an adult 

 appearance when very small, its mouth and labial rib being 

 fully formed, but when it commences to enlarge its domain 

 it cannot for some reason follow on the same axis, and the 

 renewed growth appears as a dislocation. In every speci- 

 men of the var. distorta the original labial rib is distinctly 

 visible, and I have never met with an exception. This 

 peculiarity is limited to the type form, however, whether 

 living under stones or in sea-weeds, but it never appears 

 in the var. arctica. 



Most of the Rissoa:, as well as other genera, are occasionally 

 found with a varix on the body-whorl, arising from a fresh start 

 having been made after the formation of the mouth, perhaps 

 owing to improved conditions of climate or food prompting an 

 extension of the body-whorl ; but this species and R. striatula 

 are commonly liable to these varices, whatever their cause. 

 Specimens may be found of either species having the mouth 

 and its particular sculpture fully formed, and apparently adult, 

 not one-fourth the normal size, and these would probably 

 remain as dwarfs but for some unknown change giving an im- 

 pulse to its further growth — in other words, they appear to have 

 the power either to remain dwarfed or to put on a fresh growth. 



