ART. 4 CAMBRIAN CONCHOSTRACA — ULRICH AND BASSLER 91 



MONONOTELLA, new genus 



Primitiaf (part) Matthew, Trans. New York Acad. Sci., vol. 14, 1895, p. 137. 

 Indiana (part) Matthew, Canadian Rec. Sci., vol. 8, 1902, p. 461. 



Description. — Carapace small, consisting of a single plate bent 

 sharply along the back till the edges are brought nearly together at 

 the ventral side, or possibly of two valves ankylosed along their 

 dorsal edges; ends, especially the posterior, gaping rather widely; 

 subfiisiform in a lateral view, broadly rounded posteriorly and 

 tapering anteriorly to an obtusely acuminate extremity; acuminate 

 ovate in a dorsal view; sides strongly convex, without nodes of any 

 kind. Test as in Indiana, black, thin, pliable, minutely punctate. 



Genotype. — Mononotella fusiformis {Primitiaf jusijormis Matthew). 



Remarks. — But a single species having the characters ascribed to 

 this new genus has been observed. This was described in 1895 by 

 Matthew ^^ under the name Primitiaf fusiformis. In 1902^^ the 

 same author referred the species to his genus Indiana. The generic 

 characters are taken from a nearly complete though somewhat 

 crushed example in the United vStates National Museum, which was 

 carefully compared and specifically identified with Matthew's original 

 type of the species. 



The most important addition to the knowledge of this species 

 brought out by the study of the specimens in the National Museum 

 is the apparent fact that its carapace, instead of being formed of two 

 loosely hinged valves, as in the otherwise not very dissimilar species 

 of Indiana, consists either of firmly united (ankylosed) valves or of 

 a single plate bent so as to simulate the bivalved forms. The fact 

 that the dorsal part of the test in the best specimen is not entire but 

 shows a line of fracture leaves the matter of its supposed original unity 

 still somewhat doubtful. However, the ragged nature of the line is 

 regarded as strongly indicative of separation by fracture. It cer- 

 tainly differs from the usual appearance of the line dividing the dorsal 

 edges of the valves in ordinary bivalved forms. 



Aside from the supposed dorsal ankylosis of the valves, the carapace 

 of Mononotella is not greatly different from that of Indiana. The 

 gap between the incurved edges at the ends and the ventral side is 

 much wider, and this doubtless is a valid generic character. The 

 much greater anterior type is a conspicuous peculiarity, but as only 

 a single species of Mononotella is known and as the form of the cara- 

 pace is subject to considerable variation in different species of most 

 genera of bivalved Crustacea, it is impossible as yet to decide as to 

 the degree of importance that should be ascribed to the difference. 



None of the other genera of Cambrian Crustacea described in this 

 work is closely allied. The British Hymenocaris, which has a folded, 



" Trans. New York Acad. Sci., vol. 14, 1895, p. 137, pi. 8, figs. 3a, 36. 

 " Canadian Rec. Sci., vol. 8, 1902, p. 461. 



