244 



Of this the posterior border forms a ridge, which, in the pieces 

 not perforated, extends to the anterior border in the Shark-tooth 

 form, showing its analogy to the corresponding parts in Psolus, 

 Thyonidium^ &;c. To this elevation the retractor muscles are 

 attached. 



The tentacula resemble those of Synapta, but differ in number 

 (ten instead of twelve,) and in the greater elongation of the 

 pedicel, thus allying their form somewhat to that seen in Chiro- 

 dota ; their digitations are generally eleven or thirteen. They 

 contain calcareous spiculce, consisting of a slender shaft with 

 both extremities enlarged and commonly perforated or divided. 



The genital tubes in their form, situation, and development, 

 recall those of Synapta pellucida Stimp. 



T. pallidus is of a very pale flesh color, having the walls of 

 the body so thin that the inclosed organs are seen with much 

 distinctness. It is not uncommon at Eastport, under stones, 

 near low-water mark, though it should be mentioned that under 

 the same stones were found Terebratulae, formerly considered 

 inhabitants exclusively of very deep water. 



This genus, as at present known, includes two species, T. pal- 

 lidus Ayres, and T. rotiferus [Synapta rotifer a Pourt.) It forms 

 an interesting link between Synapta and Chirodota^ though per- 

 fectly distinct from each. In general appearance it so much 

 resembles the former as to be readily mistaken for it. 



In company with the specimens of Trochinus pallidus a single 

 individual was found of an entirely different type, closely allied 

 in many respects to Thyonidium, yet varying so much that a new 

 generic division is required with the following characters. 



Body elongated, covered with suckers, among which a slight 

 tendency to separation into rows may be discerned. Tentacula 

 twenty, ramose, arranged in pairs of unequal development. 

 Oral circle of ten pieces similar to those of Thyonidium. No 

 muscular stomach. Respiratory trees two. Genital tubes undi- 

 vided, attached far backward. 



It is proposed to name the genus, from the grouping of the 

 tentacles, 



DuASMODACTYLA Ayrcs, 



the species on which it is founded being D. producta Ayres. 



