426 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



shells alone, as must necessarily be done in this instance, seems 

 impossible at this time. 



1. Small species without insertion plates or slits at margin of 



articulamentum (Family 



Lepidopleuridse) Leptochiton 



V. Valves with slitted or toothed insertion plates 2 



2. Valves in adult composed of articulamentum only, in the 



living animal entirely covered by the 



girdle Cryptochiton 



2'. Valves showing both tegmentum and articulamentum . . 3 



3. Valves with tegmental area greatly restricted, due to en- 



croachment by girdle 4 



3'. Valves with tegmental area not greatly restricted and 

 girdle not greatly encroaching 5 



4. Valves massive ; tegmentum without striking 



sculpture Katharina 



4'. Valves comparatively delicate; tegmentum sculptured 

 with a more or less scaly pattern Acanthochitona 



5. Eave tissue spongy 6 



5'. Eave tissue solid (Family Ischnochitonidae) 12 



6. Lateral areas always poorly differentiated 7 



6'. Lateral areas generally distinct, often bounded in front by 



a diagonal rib ; tail valve with a single pair of slits and 

 a posterior sinus (Family Mopaliidae, in greater 

 part) ....11 



7. Valves normally proportioned ; sculpture obsolete or of 



relatively simple type (Family Callochitonidae) .... 8 



7'. Valves narrow, of more or less bizarre form ; sculpture of 



tegmentum strongly granular Nuttallina 



8. Surface of tegmentum smooth 9 



8'. Surface of tegmentum finely granulose 10 



9. Sutural laminae and teeth very short and weakly devel- 



oped ; more than one slit on each side in median 



valves Oligochiton 



9'. Sutural laminae and teeth well developed; median valves 



with a single slit on each side Tonicella 



10. Valves stout; teeth oblique and more or less pointed in 



tail valve Cyanoplax 



10'. Valves delicate; teeth small and very numerous in end 



valves Lepidochitona 



