SCHIZOPLAX. 47 



tegmentum very spongy ; gills extending three-fourths the length 

 of the foot. 



This form, whilst allied to Tonicella, differs from all other known 

 Chitons in the curious slit along the jugum of each of the median 

 valves. This slit is wedge-shaped, narrowing to a point in front, 

 where it is lost in the spongy jugal sinus. The slit is filled with a 

 cartilaginous substance resembling the ligament of a bivalve. Out- 

 side there is a corresponding furrow not extending to the terminal 

 valves. 



The tegmentum, or external layer of shell substance, is remark- 

 ably porous where exposed at jugal sinus and eaves. 



S. BBANDTII Middendorff. PI. 11, figs. 32-37. 



Shell oval, rather elevated, the jugum rounded; olive-brown, 

 streaked, maculated or clouded with blue, sometimes chestnut. 

 Umbo central, irregularly planate. Lateral areas scarcely defined. 

 Entire surface smooth, seen under a lens to be finely granulated in 

 quincunx. Girdle narrow, olive-ashen, maculated, closely beset with 

 minute spines, but appearing almost smooth to the naked eye. The 

 median valves have a sharply cut longitudinal median sulcus. The 

 jugal sinus is moderate, deep, scarcely laminate, conspicuously 

 spongy. Eaves small, very spongy. Anterior valve with 11, pos- 

 terior 11 and median valves with 1 slit. Gills about 22, subambient. 



Length 16, width 5 mill.; divergence 140. 



Shantar Bay, Okhotsk Sea; Aleutian Is. eastward to Sitka Har- 

 bor, low water to 12 fras., on stones and shells. 



C. brandtii MIDD., Bull. Acad. Sci. St. Petersb., vi, p. 117, 1846 ; 

 Mai. Koss. i, p. 128 : Siber. Reis. p. 174, t. 15, f. 1-6. Schizoplax 

 brandti CPU. MS. DALL, Proc. U. S. Nat. Mus. 1878, pp. 2, 328. 



This very remarkable species is very prettily marbled with olive, 

 chestnut and blue ; the girdle generally dark olive, dashed with 

 ashy spots and in fine specimens having a pubescent appearance. 

 The slit is occupied by a cartilaginous substance of a dark brown 

 color, most visible from within. It is quite possible that it may 

 reach as far south as Puget Sound. 



The soft parts are yellowish-white. The gill rows extend three- 

 fourths of the length of the foot forward from their posterior termina- 

 tion, and each contains about twenty-two branchiae. Mantle-edge 

 thick, plain ; veil small, plain. Muzzle small, plain, with two large 

 squarish lappets at the posterior corners. The supposed oviducts 



