MUSEUM OF COMPARATIVE ZOOLOGY. 417 



anterior plate, and a pair at each suture between the valves. There are no 

 pores around the tail plate. The pores are very small in the fresh, and invis- 

 ible or nearly so in the dry specimen; they are filled with fine glassy spiculaj, 

 which, in the specimens I have seen, do not rise above the surface, but have 

 probably been worn off. The gills extend forward half-way to the head. The 

 muzzle is surrounded by a crumpled veil. The anus is on a papilla. I have 

 seen no spicules on the upper surface outside of the pores. The largest speci- 

 men I have seen in alcohol measures 24.0 mm. long, by 13.0 mm. wide. 



Habitat. Key West and Key Largo, Hemphill, on the reefs near low 

 water. Dry Tortugas, Dr. E. Palmer. Cape Florida, Wurdeman (in Mus. 

 Comp. Zoology). 



This remarkable species attracts the attention at once by its dark glistening 

 girdle, and long line of white streaks on its median line, like exclamation points 

 without the dots (I). With the exception of Clilamydochiton, less of the valves 

 is exposed than in any species which lias them exposed at all. A specimen 

 from which the valves had been dissolved by the acidity of the alcohol was 

 found by Dr. Carpenter in the Museum of Comparative Zoology, and doubt- 

 fully referred by him to Microplax. But we now know Microplax to be a 

 Leptoid, without slits, holding to Leptochiton much such a relation as Notoplax 

 does to Acanthochiton. 



The original type of Notoplax has the, surface of the girdle leathery, the 

 valves are wholly separated from each other by bridges of tissue, and there is 

 nothing said of sutural pores. The exposed part of the valves is larger and 

 wider, with imbricated sculpture. The slits begin as shallow furrows, which 

 are arched over higher up, and form tubes extending to the mucro, visible 

 outside as raised ridges. The valves in this species (N. speciosa) are wider 

 and shorter than in N. floridana and also farther apart, though they probably 

 overlap a little below the surface. The space in the insertion plate between 

 the two slits in the tail plate is longitudinally strongly grooved, and has a 

 serrated edge. The present species has no serration, no tubular passages to 

 the slits, and the valves are contiguous. It forms a passage from Notoplax 

 toward Stectoplax (porrectus Cpr.) in these particulars, and agrees with it in 

 having sutural pores. On the other hand, Stectoplax has the girdle covered 

 with countless minute glassy spines, the sinus is very wide, rounded, and deep, 

 and the slits are thrown forward to a very much greater extent at their distal 

 ends. The anterior and posterior laminae of insertion are strongly striated in 

 Stectoplax, and the sutural pores are conspicuous. 



It will be seen from this comparison of characters that this Florida shell is 

 intermediate in its characters. As a wider knowledge of the group may render 

 it necessary to consolidate Stectoplax with Notoplax under a more comprehen- 

 sive diagnosis of the genus, and the latter is the older name, I prefer to leave 

 this species in Notoplax, though not typical in the characters it exhibits. Its 

 place in the general grouping would be between Katharina and the Acantho- 

 chitons. The Mopaloid posterior sinus of the tail plate is practically obsolete. 



vol. xviii. 27 



