PATERINID^. 335 



Liimarsson [1876, p. 25], in describing Micromitra (Iphidella) ornatella, speaks of the pres- 

 ence of a minute foramen; but, after the study of several finely preserved ventral valves from 

 the Paradoxides beds of Andrarum, Sweden, I am led to believe that what he considered to be an 

 apical foramen is a depression in the apex of the shell on the incurved beak which was present 

 in one of the embryonic (nepionic) stages and was subsequently lost. This would involve the 

 crowding out, as it were, of the pedicle posteriorly, the only evidence of its presence in the 

 adult being the narrow, slightly depressed slit at the apex of the valve in some specimens. The 

 pedicle in the adult shell is protruded between the two valves and separated from the scar of 

 the embryonic pedicle slit by the growth of a pseudodeltidium. 



The examination of the somewhat closely allied form, Micromitra (IpJtidella) pannula 

 (White), shows the presence in some specimens of an apical depression, and its absence in 

 others. In Micromitra (Paterina) superia (Walcott), still more closely approaching the type, 

 M. (P.) iella (Billings), in external appearance, there is no trace of an apical slit. In Micromitra 

 {Paterina) crenistria (Walcott) , which is associated with M. (P.) superba, the apex is rounded, while 

 in the somewhat similar but distinct form, M. pealei (Walcott), from the Gallatin River valley of 

 Montana, the apical slit is as clearly shown as in Micromitra {Iphidella) ornatella (Linnarsson) , 

 but there is no apical foramen. 



Hall and Clarke [1892c, p. 97] illustrate a specimen of Micromitra {Paterina) hella showing 

 an apical foramen, but in the description of PI. IV, fig. 8, the statement is made that the foramen 

 is not altogether distinct in the specimen. 



The systematic position of the genus Micromitra appears to be in the Atremata of Beecher 

 [1891, p. 354], with possible characters that nearly place it in the Protremata. The presence 

 of an obscure apical furrow in front of the apex of Micromitra {Paterina) hella (Billings), Micro- 

 mitra (Iphidella) ornatella (Linnarsson), and in some specimens of Micromitra pealei (Walcott) 

 tends to support the view that tlie pedicle was apical in its early stages. We also have to con- 

 sider the area of what appears to be the dorsal valve of Micromitra {Iphidella) pannula. In 

 this species there is a rather deep convex pseudodeltidium in the conical valve and a clearly 

 defined, depressed pseudodeltidium on the flat valve between the sides of the narrow false 

 area. There is also present on the latter valve a narrow median groove extending from the 

 apex to the posterior margin of the pseudodeltidium; it is similar in appearance to the pedicle 

 groove of the ventral valve of Obolus and Bhinoholus. This suggests that the fiat valve may be 

 the pedicle valve, but the presence of a scar of the embryo pedicle opening on the conical valve 

 is opposed to such an interpretation. 



It may be urged that Micromitra is a true atrematous genus, the pedicle never having been 

 inclosed and the slit on the ventral valve being only a depression without special significance. 

 This may be so, but the impression it conveys to me is that the slit indicates a pedicle opening 

 which existed near the apex of the ventral valve at an early stage in its growth, but which was 

 subsequently closed, the pedicle continuing to protrude between the valves. 



Whiteaves wrote me, under date of July 23, 1896, that with the exception of the specimen 

 which is referred in tliis monograph to Micromitra {Paterina) logani (see pp. 345 and 350), 

 there were no specimens or electrotypes of the types of Micromitra {Paterina) Iella (Billings) in 

 the collections of the Geological Survey of Canada, nor of the closely allied species of the same 

 genus from Topsail Head, mentioned by Billings [1872b, p. 478]. Under the circumstances 

 nothing can be done but to decide from Billings's description and figure [1872b, p. 477] the 

 characters of the species, and refer to the species the form that can best be identified with the 

 description and figure as its typical representative. This appears to be the form from 

 the Lower Cambrian limestone of York County, Pennsylvania. (See pp. 345 and 350, and 

 PI. II, figs. 1 and Ic.) 



Ornamentation of the surface. — One of the most noticeable characters is the surface orna- 

 mentation. The type species of Micromitra {M. sculptilis (Meek), PI. Ill, figs. 5, 5a-c) has 

 on some specimens (PL III, figs. 5c, 5d) concentric lines similar to those on the less ornamented 

 forms of the genus, as represented by Micromitra {Paterina) labradorica (Billings), and on other 

 specimens a well-defined series of radiating ridges of varying number, width, and strength 

 (PL III, figs. 5, 5a, and 5c). It was on account of the surface produced by the concentric lines 

 crossing the radiating ridges that the specific name sculptilis was proposed. 



