288 Mr. W. H. Dall on a Natural Arrangement 



Family Acmseidse, Carpenter. 



Shell patelliform. Animal provided with a free cervical 

 branchia issuing from the left side of the body, above the 

 head ; muzzle surrounded with a frill of integument. Radula 

 without a rhachidian tooth, and with three lateral teeth on 

 each side ; with or without accessory uncini. 



A. Destitute of a branchial cordon. Acmcea. 



1. Acmcea, Eschscholtz, 1828. (Syn. Tecture, Cuvier, 1830 ; 



Tectura, Gray, 1847.) 

 Teeth subequal, parallel in both axes ; uncini absent ; 

 muzzle-frill produced into two lappets. 

 Formula, ? 



' 0(1—1—1 . 1—1—1)0 



Tjrpe A. mitra, Esch. Zool. Atlas, 1833, v. p. 18. no. 1 ; 

 Philippi, Zeit. f. Mai. 1846, p. 106. 



2. CoUisellaj Dall, n. subgen. 



a. Third lateral smaller than, and opposed to, the second ; 

 first laterals anterior ; muzzle-frill without lappets ; 

 a single minute uncinus on the pleura. 

 Formula, 



1 (2—1 . 1—2) 1 



Type Acmwa ]jeltaj Esch. I. c. 1833, no. 5. 

 b. Provided with two minute uncini on the pleura. 



(? ColUsellina). 

 Formula, " 



' 2(2—1 . 1—2)2 



Type Patella saccliarina^ Linn., Gmel. S. N. 1792, 

 p. 3695. no. 19. 



B. Cordon present, interrupted in front. Lottta. 



1. Lottia (Gray), Cpr. 1863. Without muzzle-lappets; teeth 

 as in ColUsella (a). 







Formula, — 



' 1 (2—1 . 1—2) 1 



Type L. gigantea (Gray), Cpr. Am. Journ. Conch. 1866, 

 ii. p. 342. 



C. Cordon present, complete, uninterrupted. Scurria. 



1. Scurriaj Gray, 1847. No muzzle-lappets; teet has in the 

 last. 

 Formula, ? 



' 1 (2—1 . 1—2) 1 



Types : S. scurra (Lesson), Gray, P. Z. S. 1847, p. 158 ; 

 8. mesoleuca (Mke.), Cpr. Maz. Cat. 1857, p. 208. 

 no. 263 (as Acmcea). 



