66 ISCHNOCHITOX. 



or undulating. Toward the outer edges the wrinkles become 

 coarser and more separated. End valves sculptured with radiating 

 riblets ; posterior valve having the mucro central. Interior white, 

 the end valves usually marked with a brown crescent ; sutural lobes 

 rounded ; sinus wide and flat. Anterior valve having 12, median 

 valves 1, posterior valve 12 slits ; teeth smooth, rather sharp. 

 Eaves solid. 



Girdle covered with flattened striated scales, which are very 

 unequal in size, but become larger toward the valves (fig. 21). 



Length 32, breadth 16 mill. 



Australia. 



The texture of the shell is peculiar, the outer layer being thin 

 and easily broken through, exposing an extremely spongy layer. 

 Carpenter's specimens seem to have been worn so as to expose this 

 layer in places, hence the name cariosus. In the normal and un- 

 worn specimens before me the outer surfare is scarcely carious 

 though dull and uneven, nor are the eaves spongy. Carpenter's 

 original description is as follows : 



Heterozona cariosa (pi. 24, fig. 23). Shell subelongate, rather 

 elevated, the jugum rounded. Mucro median, moderately elevated ; 

 whitish-ashen, slightly variegated at the sutures occasionally. 

 Central areas very granose in a young specimen, the granules 

 minute and close toward the jugum, then becoming rugose lines, 

 undulating anteriorly, and widely separated on the sides; the adult 

 shell is carious. Lateral areas having 3-8 deeply but irregularly 

 granose radii, the grains at the sutures large ; end valves having 

 30-40 graniferous radii. Interior : posterior valve with 11, central 

 1, anterior valve 11 slits. Teeth acute; eaves wide, spongy; sinus 

 large, flat, smooth. Girdle (pi. 24, fig. 23), clothed with noriial 

 striated imbricating scales and having long, large striated solid 

 scales scattered here and there. (Cpr.~) 



Length 30, breadth 16J mill., divergence 100. 



Australia (Mus. Cuming No. 46). 



This shell forms an exact transition between Ischnoplax and the 

 true Ischnochiton. The mantle resembles Ischnoplax in its double 

 series of scales ; although if the large ones were plucked out, the 

 latter would be nearly of a normal arrangement, except that they 

 are rather narrow as in /. castus. It differs from Ischnoplax, not 

 only in not being narrow and elongated, but in having a normal 

 mucro. The species is curiously like Stenoradsia magdalenensis, 



