Throughout these papers Roman numerals will be used to denote the segments of the earthworms described (e.g. xvii = segment seventeen; xvii/xviii = septum dividing xvii and xviii) and external intersegmental grooves will be denoted by ordinary numerals, e.g. 7/8, 8/9, etc.

One of the most common genera of earthworms occurring in the Australasian region is the genus Rhododrilus of the sub-family Acanthodrilinae. Nine species of the genus have been described by previous workers, three from the North Island of New Zealand, one from Little Barrier Island, one from the Kermadec Islands, one from Chatham Islands, one from Queensland, and two from the sub-antarctic islands of the New Zealand region. Eleven new species of the genus, all from the North Island of New Zealand, are described in this paper.

The genus was established by Beddard (1889) to include species showing the following characters:

Eight chaetae per segment. Nephridiopores in a single series on each side. Male pores on xviii or xvii or in 17/18; one pair of prostatic pores on xvii. Spermathecal pores one to four pairs, the last pair in 8/9. Gizzard well developed. Two pairs of free testes and funnels in × and xi.

It shows close affinities to Microscolex, from which it differs principally in the possession of a well-developed gizzard (in Microscolex the gizzard is vestigial). Michaelsen (1900) distinguished the two genera, but in 1905 he united Rhododrilus and Microscolex and then in 1907 separated them again and they have remained separate since then. Benham (1909) established a genus Leptodrilus for two species from Auckland and Campbell Islands which are identical with Rhododrilus except that the male and prostatic pores are on xvi instead of xvii. Michaelsen (1923) states, “[this difference] I regard as an abnormality of little systematic moment; in my opinion it is even doubtful whether there is any specific difference between Leptodrilus leptomerus and Rhododrilus cockayni, let alone a difference in genus.” Stephenson (1930) allows the genus Leptodrilus to stand, but it is doubtful whether the differences from Rhododrilus are significant.

One of the species described in this paper (R. disparatus) shows characters atypical of the genus in possessing a small ventro-lateral cluster of micronephridial tubules beside each of the meganephridial tubules in ii–xx. Also in R. intermedius, described in this paper, the ventro-lateral portions of the meganephridial tubules are coiled in a spirillar form, superficially resembling micronephridia. This condition is similar to that found by Benham and Cameron (1913) in Plagiochaeta (Perieodrilus) ricardi and P. montanus. It is regarded by Stephenson (1930) as probably intermediate between the meganephridial and the micronephridial conditions. There is little to distinguish R. disparatus (and to a lesser extent R. intermedius) from the sub-family Octochaetinae, especially since the author has recently discovered a new genus of that sub-family which has the microscolecine arrangement of the male reproductive organs (this genus will be described in a later paper).

The New Zealand species of Rhododrilus may be identified from the following key:

R. robustus, by its possession of five pairs of spermathecae, falls outside the definition of the genus given by Stephenson (1930), but this character is not significant and the definition of the genus may be extended to include species having more than four pairs of spermathecae.

The following are descriptions of eleven species of Rhododrilus discovered in the North Island of New Zealand. Type specimens in the author's collection.

Rhododrilus albidus n.sp. (Plate 104, figs. 1–3)

Twenty specimens of this slender, unpigmented earthworm were collected from Taupo sandy silt and Puketitiri sandy silt in Ball's Clearing Forest Reserve, Puketitiri, Hawke's Bay. In its external features the species is rather similar to R. sutherlandi, described by the author from the “gumlands” of North Auckland.

The specimen on which this description is based is 95 mm. in length and 2·75 mm. in diameter, with 119 segments.

The prostomium is tanylobous. The chaetae are eight per segment, arranged in pairs. On xxiv:

ab = cd = 0·5 mm.; aa = 1 mm.; bc = 1·5 mm.; dd = 2 mm.