264 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 10 



Notomastus magnus Hartman 



Hartman, in press, AHF. 



This occurs in Tomales Bay, in sand flats off Pelican Point. It is 

 bluish red in life, and intermediate in size and thickness between Dasy- 

 branchus lumbricoides (above) and the very slender N. tenuis (below). 

 Length is 70 to 100 mm. It is described in another report (Hartman, in 

 press, AHF). 



Notomastus tenuis Moore 



Moore, 1909, pp. 277-278, pi. 9, fig. 55. 

 N. angulatus Chamberlin, 1919, pp. 16-17. 



This is very common in sand and mud flats over a wide range of 

 intertidal area, including Tomales Bay, Bodega Bay, Tomales Point, 

 ocean side, and in holdfasts of Phyllospadix. It is bluish red in life, and 

 very long, extremely slender, attenuate, fragile. It is readily distinguished 

 by its color from another fragile species, Scoloplos acmeceps (see above), 

 with which it sometimes occurs. Length is 100 to 250 mm. I am unable 

 to distinguish N. tenuis from A^. angulatus Chamberlin (1919). Among 

 large collections the relative rugosity of segments and the degree of thick- 

 ness of thorax and abdomen show all possible gradations. 



Genus MEDIOMASTUS, new genus 



The thorax consists of 1 1 seginents, including a well-developed, 

 achaetous peristomial ring; segments 2 to 5 have pointed limbate setae 

 only, and segments 6 to 11 have long-handled uncini. The abdomen has 

 numerous segments, provided with long-handled uncini only. It differs 

 from Heteromastus Eisig in having 11, instead of 12, thoracic segments, 

 and in lacking the reticulations on the cuticle; it differs from Capito- 

 mastus Eisig in having normal hooks on the last segments and in lacking 

 the heavy, copulatory hooks on the last thoracic segments. 



Type of the genus is A^. calif orniensis, new species. 



Mediomastus californiensis, new species 

 Plate 26, Figs. 64, 65 



Hartman, in press, AHF. 



Length attains 25 mm ; width is less than 1 mm. This is a very slender, 

 threadlike form, thickest in the region of the second and third setigers; it 

 consists of 100 or more segments, each of which is about as long as, or 

 longer than, wide, save the first few rings. Segments are typically uni- 

 annulate except for ridges that mark the setigerous processes. The long 

 posterior end, including most of the abdominal segments, is more or less 

 closely coiled in fixative, but the single segments are moniliform. 



