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Shell about 3.5 mm long; found crawling on algae in a low intertidal pool at McKenney Point, Cape Elizabeth.

See:
Ohnheiser & Malaquias (2014. The family Diaphanidae (Gastropoda: Heterobranchia: Cephalaspidea)
in Europe, with a redescription of the enigmatic species Colobocephalus
costellatus M. Sars, 1870. Zootaxa 3774 (6): 501–522.) Available from: https://www.researchgate.net/publication/260684868

https://opistobranquis.info/en/guia/cephalaspidea/cylichnoidea/diaphana-minuta/

Body like a shelled slug, to 33 mm long. Foot elongate with bilateral, rounded extensions which it can use to swim in a flapping motion. This snail is primarily a benthic species that lives in soft sediments and is not considered a truly pelagic animal (see Wrobel & Mills, 1998). Found swimming mostly in the winter months.

I believe it is Philinorbis acuticauda - it looks similar in the description and the animal was between 5-10 cm long! My husband took that video showing the animal diving into the sand! https://youtu.be/7tAlGzFvKxE

Philinorbis only lives in the deep sea, this animal however was found in less then 20 m. I added a few pictures taken from the video.

12 mm long in 1st image; 20 mm in 2nd; specimen in 3rd image in situ. All were found under the Hwy 101 bridge over Richardson Bay. 4th image is of egg mass found on the same date; 5th image is close-up of strand from within the egg mass.

6th image is of newly hatched larvae trapped in surface film of water (shells 370 microns long). 7th image is ventral view of newly hatched larvae crawling on underside of coverslip. Both of these latter images were taken on 27 Oct 2013.