+--------------+--------------+--------------+------------------+ | |Basal diameter|Basal diameter| Medium sized | | | of shell | of shell | specimen about | | | 1/20th of | 1/5th of |3/4ths of an inch | | | an inch. | an inch. |in basal diameter.| +--------------+--------------+--------------+------------------+ |First cirrus }| ? | 11 | 17 | |shorter ramus}| | | | | | | | | |Second cirrus | 4 or 5 | 9 | 13 | | | | | | |Sixth cirrus | 9 or 10 | 19 |31, in another 36 | +--------------+--------------+--------------+------------------+ In the specimen 1/5th of an inch in basal diameter, each segment of the posterior cirri carried five pairs of spines; whereas, in full-grown specimens, there are six or seven pairs. In the 1/20th of an inch specimen, on the inner maxillæ, there were no spines between the upper large and the lower large pair of spines; whereas, in the 1/5th of an inch specimen, there were five intermediate spines, and in larger specimens nine or ten spines. Notwithstanding the difficulties now enumerated, I hope that, owing to having examined a vast number of specimens of the most varying species, I have not fallen into very many errors. I have endeavoured to err on the side of making too few instead of too many species. In those cases, however, in which I have seen only a few specimens, I have been sometimes compelled to decide without sufficient evidence.